Ambiguous citations

Thank you in advance for any advice on this (probably very simple) issue.

I am using Harvard and desktop Endnote

I have quite a few citations that Endnote is indicating are ambigous (eg. Smith et al 2005a) - but it isn’t an ambiguous citation - there aren’t any other “Smith et al 2005” citations in the library.

I’ve checked my trash and there are none in there.

Why is it doing this and how can I fix it?