Yeah like DS9 having repeat characters makes sense because the station isn't going anywhere. How the heck do the same people show up on Voyager?

This has already been kinda answered.

They showed how they do it.
Each civilization had their own agreements and there was a lot of zig-zagging and going around stuff.

Unless the producers/audience reception is nonplussed, as with the Vidiians or the Kazon. At which point they fall back on old reliable Borg.

Then there was the time Naomi Wildman's mom got left on some planet somewhere and everyone forgot she existed.

Edit: This gives me an idea.

Like that cardadian Who had surgery to look like a bajorin showing up like three seasons later.