Duplicate screen dictionary keys when generating YAML files

[kabanaty]

Describe the bug
The function extract_screens in generate.py contains two dictionary keys titled "image". I believe this implements a sort of race condition that sets the "image" PV to whichever of the two possibilities returns first.

To Reproduce
Run write.py and compare the output - see DL1.yaml.

Expected behavior
Two possible PVs should have different names for dictionary keys.