[Elwynd02]

Try this, to replicate:
1.Take 2 books and give them both these two tags: fantasytest, test1.
2.Do a bulk download of metadata for the two books.
3.Click to review the downloaded metadata
4.On book 1, click the arrow to bring across the old tags (fantasytest, test1). This will correctly overwrite what is in the left box, but this hasn't tested the problem yet.
5.Now, in the left box, remove test1, so you're just left with fantasytest in the left box
6.Click next to go book 2.
7.For this one, just accept whatever the download suggested for the tags, ie don't try to put the existing tags on it. The left tag field will show whatever the download suggested. (If you typed in a completely different tag instead, eg test2, this would also demonstrate the issue.)
8.Click next so it writes the data on those two books.
9.Look at the tags on the books. For me, book 1 still says "fantasytest, test1", and book 2 says "Romance, Adult, fantasytest, test1". (If I'd typed in test2 instead of any other tags in step 7, it would say "fantasytest, test1, test2".)