Avoid duplicate suppress warning suggestions through new API method.

[rgrunber]

• See Fix duplicate add suppress warnings proposals eclipse-jdt/eclipse.jdt.ui#1942
• Implement alreadyHasProposal to avoid duplicates

I don't think we are seeing these duplicate entries, so it's worth me having a quick look to see why that is.

Update:

It's because of

eclipse.jdt.ls/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/handlers/CodeActionHandler.java

Lines 192 to 194 in 78d72ee

	Set<ProposalKindWrapper> quickSet = new TreeSet<>(comparator);
	quickSet.addAll(quickfixProposals);
	proposals.addAll(quickSet);

eclipse.jdt.ls/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/handlers/CodeActionHandler.java

Lines 406 to 425 in 78d72ee

	private static class ChangeCorrectionProposalComparator implements Comparator<ProposalKindWrapper> {
	@Override
	public int compare(ProposalKindWrapper p1, ProposalKindWrapper p2) {
	String k1 = p1.getKind();
	String k2 = p2.getKind();
	if (!StringUtils.isBlank(k1) && !StringUtils.isBlank(k2) && !k1.equals(k2)) {
	return k1.compareTo(k2);
	}
	int r1 = p1.getProposal().getRelevance();
	int r2 = p2.getProposal().getRelevance();
	int relevanceDif = r2 - r1;
	if (relevanceDif != 0) {
	return relevanceDif;
	}
	return p1.getProposal().getName().compareToIgnoreCase(p2.getProposal().getName());
	}
	}

We remove duplicates on our end also