Some files have the same tag twice - is it possible to delete one?

[so_la_la]

Hi,
I have a tag written into my files indicating the popularity on last.fm. Apparently i have some of them twice, i.e. in the file inspector pane I have two lines with the tag and a value each.
Is it possible to delete one of them but not the other?
I have tried with simply selecting all files but that won't do the trick.
Thanks.