I was curious because my juice is the same flavor. It's just my nicotine levels are different 15mg and 16mg. Old is 15, New is 16.
That is so little difference, it won't matter one iota. If the mg's were quite different, they would average. You'll end up with 15.5mg, if there is an equal amount of both levels.
What they all said about mixing flavors is true. But even if the flavor is the same... it can also be a case of, is the juice already in there, gunked at all? If you vape something sweet, or use a lot of flavoring, the coil and wick can get gunked up, and the gunk can backwash into the tank or well -- mine is really bad to do that. So I'll put new juice on top of old juice, for a while when a wick is new, but once the juice that's in there starts looking dark, I dump it and pull the wick, dry-burn the coil, and insert a fresh wick.
Andria