[Dx4win] merge question

[Steve K7AWB]

A friend has his DX4WIN log which may be missing several thousand QSOs due to an error in a previous competitive program (made before the transfer to DX4WIN as the new main logging program now).  I have a special adif download from LOTW which may include those missing QSOs.  How can I merge the downloaded file into the main log, but prevent duplicates?  I made a special DX4WIN log of the 50K+ QSOs adif's for an easy merge. 

What defines a duplicate QSO from a non-duplicate QSO?  I mean how many parameters must match to be a dup?

73

Steve K7AWB