Why are questions allowed to have two similar tags?
https://stackoverflow.com/posts/3726446/revisions, revision 5 or earlier.
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Contrary to the answers posted so far tagging with both tags is correct.
.net3.5tag is needed to indicate that either the question is about a specific feature of the version 3.5 of the .NET framework or to indicate that the user is using that specific version, and thus solutions that are not compatible with it are unacceptable.
.nettag is also needed for several reasons:
.netbadge which indicates a knowledge about the entire .NET framework, including the specific 3.5 version
[.net] textinstead of
.net tag is for questions generally concerning the .NET Framework, independent of the version, while the
.net3.5 tag should indicate a question is specific to that version. In this case I guess the OP wasn't certain whether his question applies to all versions or not, someone knowing should adequately retag.