The duplicate tag helps to identify possible duplicate proposals.

When you think that a proposal might be a duplicate of another one, you can tag your question with the tag.

A proposal might be considered as duplicate if it's a subtopic of a more general existing proposal, or it would drain from an existing site.

for example, a C# or Java proposal should be considered as a duplicate of StackOverflow as this site has specific tags in order to handle questions related to C# and Java.