Any Advice?

I have heard of mint and proof set that include duplicates like that. Some people might pay a few cents more for the packaging error, but most people seem to want the correct coins in there set.

I would actually say that a set that has a duplicate (with one missing coin) is worth less than a good set.

I recently purchased a 2002 uncirculated State Quarter set and was looking at it today when I discovered that the set has 2 tennessee coins!? What does this mean exactly? did it come from the mint like that or was it tamperered with?it doesn't look like it has. does this decrease the value of the set? I'm new to buying sets so any help and/or advice is appreciated.

Please be more specific. Were the sets in two separate plastic holders or was the duplication in the same plastic holder? Not much sence in tampering with that set, either. Not worth enough to replace anything with anything. Sounds like just a Mint error. If so, would be a keeper for a conversation piece with other collectors.
Of course you may have run into that rare and new North and South Tennessee state split.