diff --git a/vlib/v/ast/types.v b/vlib/v/ast/types.v
index d75ce7025b1b56..381a395a5b57b4 100644
--- a/vlib/v/ast/types.v
+++ b/vlib/v/ast/types.v
@@ -250,6 +250,8 @@ pub mut:
 
 @[minify]
 pub struct SumType {
+pub:
+	attrs []Attr
 pub mut:
 	fields       []StructField
 	found_fields bool
@@ -262,6 +264,17 @@ pub mut:
 	parent_type    Type
 }
 
+// used to check whether there a generic type which is duplicate, returns array with no duplicates
+pub fn (st &SumType) get_deduplicated_variants() []Type {
+	mut variants := []Type{}
+	for v in st.variants {
+		if v !in variants {
+			variants << v
+		}
+	}
+	return variants
+}
+
 // <atomic.h> defines special typenames
 pub fn (t Type) atomic_typename() string {
 	idx := t.idx()
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 2aaf3593f419c9..12dfc2f7982844 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -3082,6 +3082,37 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 			ft := c.table.type_to_str(from_type)
 			tt := c.table.type_to_str(to_type)
 			c.error('cannot cast `${ft}` to `${tt}`', node.pos)
+		} else if to_sym_info.variants.len != to_sym_info.get_deduplicated_variants().len {
+			if !to_sym_info.attrs.any(!it.has_arg && it.name == 'ignore_generic_duplicates') {
+				// TODO: not a option type because of autofree bug; see https://github.com/vlang/v/issues/20937
+				mut message := '<no message was defined>'
+				mut message_type := 'error'
+				for attr in to_sym_info.attrs {
+					if attr.name == 'on_generic_duplicate' && attr.has_arg {
+						message = attr.arg
+					}
+				}
+				if message.starts_with('error: ') {
+					message = message[7..]
+				} else if message.starts_with('warn: ') {
+					message_type = 'warn'
+					message = message[6..]
+				} else if message.starts_with('notice: ') {
+					message_type = 'notice'
+					message = message[8..]
+				} else if message.starts_with('note: ') {
+					message_type = 'notice'
+					message = message[6..]
+				}
+				if message.len > 0 {
+					match message_type {
+						'warn' { c.warn('duplicate type: ${message}', node.pos) }
+						'notice' { c.note('duplicate type: ${message}', node.pos) }
+						'error' { c.error('duplicate type: ${message}', node.pos) }
+						else {}
+					}
+				}
+			}
 		}
 	} else if mut to_sym.info is ast.Alias && !(final_to_sym.kind == .struct_ && final_to_is_ptr) {
 		if (!c.check_types(from_type, to_sym.info.parent_type) && !(final_to_sym.is_int()
diff --git a/vlib/v/checker/tests/message_on_duplicate_using_generic.out b/vlib/v/checker/tests/message_on_duplicate_using_generic.out
new file mode 100644
index 00000000000000..f6dbf100faced3
--- /dev/null
+++ b/vlib/v/checker/tests/message_on_duplicate_using_generic.out
@@ -0,0 +1,34 @@
+vlib/v/checker/tests/message_on_duplicate_using_generic.vv:21:7: notice: duplicate type: blah blah blah 22222
+   19 | fn main() {
+   20 |     _ := Name1[int](2)
+   21 |     _ := Name2[int](4)
+      |          ~~~~~~~~~~~~~
+   22 |     _ := Name3[int](6)
+   23 |     _ := Name4[int](8)
+vlib/v/checker/tests/message_on_duplicate_using_generic.vv:22:7: warning: duplicate type: blah blah blah 33333
+   20 |     _ := Name1[int](2)
+   21 |     _ := Name2[int](4)
+   22 |     _ := Name3[int](6)
+      |          ~~~~~~~~~~~~~
+   23 |     _ := Name4[int](8)
+   24 |     _ := Name5[int](9)
+vlib/v/checker/tests/message_on_duplicate_using_generic.vv:20:7: error: duplicate type: blah blah blah 11111
+   18 |
+   19 | fn main() {
+   20 |     _ := Name1[int](2)
+      |          ~~~~~~~~~~~~~
+   21 |     _ := Name2[int](4)
+   22 |     _ := Name3[int](6)
+vlib/v/checker/tests/message_on_duplicate_using_generic.vv:24:7: error: duplicate type: blah blah blah 55555
+   22 |     _ := Name3[int](6)
+   23 |     _ := Name4[int](8)
+   24 |     _ := Name5[int](9)
+      |          ~~~~~~~~~~~~~
+   25 |     _ := Name6[int](10)
+   26 | }
+vlib/v/checker/tests/message_on_duplicate_using_generic.vv:25:7: error: duplicate type: <no message was defined>
+   23 |     _ := Name4[int](8)
+   24 |     _ := Name5[int](9)
+   25 |     _ := Name6[int](10)
+      |          ~~~~~~~~~~~~~~
+   26 | }
diff --git a/vlib/v/checker/tests/message_on_duplicate_using_generic.vv b/vlib/v/checker/tests/message_on_duplicate_using_generic.vv
new file mode 100644
index 00000000000000..9e2753f64ed6d0
--- /dev/null
+++ b/vlib/v/checker/tests/message_on_duplicate_using_generic.vv
@@ -0,0 +1,26 @@
+@[on_generic_duplicate: 'blah blah blah 11111']
+type Name1[T] = T | int | string
+
+@[on_generic_duplicate: 'notice: blah blah blah 22222']
+type Name2[T] = T | int | string
+
+@[on_generic_duplicate: 'warn: blah blah blah 33333']
+type Name3[T] = T | int | string
+
+@[on_generic_duplicate: 'note: blah blah blah 44444']
+@[ignore_generic_duplicates]
+type Name4[T] = T | int | string
+
+@[on_generic_duplicate: 'error: blah blah blah 55555']
+type Name5[T] = T | int | string
+
+type Name6[T] = T | int | string
+
+fn main() {
+	_ := Name1[int](2)
+	_ := Name2[int](4)
+	_ := Name3[int](6)
+	_ := Name4[int](8)
+	_ := Name5[int](9)
+	_ := Name6[int](10)
+}
diff --git a/vlib/v/gen/c/auto_str_methods.v b/vlib/v/gen/c/auto_str_methods.v
index 62e640fe5e5437..bf33e637f2b67f 100644
--- a/vlib/v/gen/c/auto_str_methods.v
+++ b/vlib/v/gen/c/auto_str_methods.v
@@ -427,7 +427,7 @@ fn (mut g Gen) gen_str_for_union_sum_type(info ast.SumType, styp string, typ_str
 		clean_sum_type_v_type_name = util.strip_main_name(typ_str)
 	}
 	fn_builder.writeln('\tswitch(x._typ) {')
-	for typ in info.variants {
+	for typ in info.get_deduplicated_variants() {
 		typ_name := g.typ(typ)
 		mut func_name := g.get_str_fn(typ)
 		sym := g.table.sym(typ)
diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 27ca5faa34d01e..4637b5051bd59f 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -925,7 +925,7 @@ pub fn (mut g Gen) write_typeof_functions() {
 			g.writeln('${static_prefix}char * v_typeof_sumtype_${sym.cname}(int sidx) { /* ${sym.name} */ ')
 			if g.pref.build_mode == .build_module {
 				g.writeln('\t\tif( sidx == _v_type_idx_${sym.cname}() ) return "${util.strip_main_name(sym.name)}";')
-				for v in sum_info.variants {
+				for v in sum_info.get_deduplicated_variants() {
 					subtype := g.table.sym(v)
 					g.writeln('\tif( sidx == _v_type_idx_${subtype.cname}() ) return "${util.strip_main_name(subtype.name)}";')
 				}
@@ -934,7 +934,7 @@ pub fn (mut g Gen) write_typeof_functions() {
 				tidx := g.table.find_type_idx(sym.name)
 				g.writeln('\tswitch(sidx) {')
 				g.writeln('\t\tcase ${tidx}: return "${util.strip_main_name(sym.name)}";')
-				for v in sum_info.variants {
+				for v in sum_info.get_deduplicated_variants() {
 					subtype := g.table.sym(v)
 					g.writeln('\t\tcase ${v.idx()}: return "${util.strip_main_name(subtype.name)}";')
 				}
@@ -946,7 +946,7 @@ pub fn (mut g Gen) write_typeof_functions() {
 			g.writeln('${static_prefix}int v_typeof_sumtype_idx_${sym.cname}(int sidx) { /* ${sym.name} */ ')
 			if g.pref.build_mode == .build_module {
 				g.writeln('\t\tif( sidx == _v_type_idx_${sym.cname}() ) return ${int(ityp)};')
-				for v in sum_info.variants {
+				for v in sum_info.get_deduplicated_variants() {
 					subtype := g.table.sym(v)
 					g.writeln('\tif( sidx == _v_type_idx_${subtype.cname}() ) return ${int(v)};')
 				}
@@ -955,7 +955,7 @@ pub fn (mut g Gen) write_typeof_functions() {
 				tidx := g.table.find_type_idx(sym.name)
 				g.writeln('\tswitch(sidx) {')
 				g.writeln('\t\tcase ${tidx}: return ${int(ityp)};')
-				for v in sum_info.variants {
+				for v in sum_info.get_deduplicated_variants() {
 					g.writeln('\t\tcase ${v.idx()}: return ${int(v)};')
 				}
 				g.writeln('\t\tdefault: return ${int(ityp)};')
@@ -6399,13 +6399,16 @@ fn (mut g Gen) write_types(symbols []&ast.TypeSymbol) {
 				struct_names[name] = true
 				g.typedefs.writeln('typedef struct ${name} ${name};')
 				g.type_definitions.writeln('')
+
 				g.type_definitions.writeln('// Union sum type ${name} = ')
-				for variant in sym.info.variants {
+				variants := sym.info.get_deduplicated_variants()
+				for variant in variants {
 					g.type_definitions.writeln('//          | ${variant:4d} = ${g.typ(variant.idx()):-20s}')
 				}
 				g.type_definitions.writeln('struct ${name} {')
 				g.type_definitions.writeln('\tunion {')
-				for variant in sym.info.variants {
+
+				for variant in variants {
 					variant_sym := g.table.sym(variant)
 					mut var := variant.ref()
 					if variant_sym.info is ast.FnType {
diff --git a/vlib/v/parser/parser.v b/vlib/v/parser/parser.v
index d47fc90cc0e649..6b4b5186e03cf7 100644
--- a/vlib/v/parser/parser.v
+++ b/vlib/v/parser/parser.v
@@ -4373,6 +4373,7 @@ fn (mut p Parser) type_decl() ast.TypeDecl {
 			cname: util.no_dots(prepend_mod_name)
 			mod: p.mod
 			info: ast.SumType{
+				attrs: p.attrs
 				variants: variant_types
 				is_generic: generic_types.len > 0
 				generic_types: generic_types
diff --git a/vlib/v/tests/ignore_duplicate_using_generic_test.v b/vlib/v/tests/ignore_duplicate_using_generic_test.v
new file mode 100644
index 00000000000000..3cdee14ceee01a
--- /dev/null
+++ b/vlib/v/tests/ignore_duplicate_using_generic_test.v
@@ -0,0 +1,15 @@
+@[ignore_generic_duplicates]
+type Name[T] = T | int | string
+
+fn f(n Name[string]) {
+	assert n as string == 'lolol'
+}
+
+fn g(n Name[int]) {
+	assert n as int == 123
+}
+
+fn test_main() {
+	f(Name[string]('lolol'))
+	g(Name[int](123))
+}