How do I go about downloading a second copy of an earlier retrieved file? When I try, I get a msg. telling me it is a duplicate request, and then asking if I want to merge this with the earlier one? I've tried answering with both YES and NO and I can't tell anything happens either way. My first download was lost so I need a replacement. How do I set Tixati to let this happen?

If it asks you about a duplicate transfer, it means the link you added IS EXACTLY the same.
In essence, if your old transfer has no seeds AND you lost your file, this one is completely lost. You would need to find a completely different transfer that included that same file (a bitwise perfect copy).

Tixati wouldn't ask you with a duplicate transfer prompt in the following cases:
1) The same file is transferred using a different file name within or a different piece size
2) Same torrent but with private flag set/unset (different to whatever you have)
3) The same file is distributed in a torrent transfer among other files

Because all of the above causes the info-hash to change (aka the magnet-link portion that points to the data). You really do need to find a different transfer to your old one. Good luck. Try out official sources if these exist (look up their website on archive.org) or try the DHT search engines, maybe you get lucky.