If you have added the same image in multiple catalogs and the catalogs use the same Smart Collection's, then yes, they will appear in multiple catalogs.
Btw: You can have Smart collection as template's that are automagically added to every new catalog you're creating.
Out of curiosity: Why are you using multiple catalog's?
I have pics from about a dozen weddings in Lightroom and have made the mistake of starting a new catalogue for each one
Having just gone through and identified some 5 star shots from each wedding ( now in smart collections in each catalogue )
I wondered if it was possible to get them into one collection ???
Just an idea: You can import a catalog into another catalog. This way, you could merge all catalogs to one and add some Tags upon import to easily being able to still tell the sources apart.
heh thanks very much - will have a shot at that.....