There's always going to be a difference of opinion. I've seen all types, as I'm sure a lot of you have. The guy who never wants to rehearse, and just wants to "wing it" out there on stage and baffle the audience with B.S. Then there's the guy who thinks it should take a year to have 25 songs down note for note, exact, perfect, duplication of the original recording in every way. And everything in between. I never want to play with either of the first two guys. But I do agree with Nev375 in that there is only just so far you can go in rehearsal. Sooner or later, you gotta get out there and lay it on the line. You can rehearse day after day, night after night, have hundreds of songs you know, but it isn't until you actually get out and play them for a crowd that you find out what your band is made of. I don't think it's a terrible thing to go out and do a show when you're "just about ready". It will show you what you've got. Which is way better than spending a huge amount of time in rehearsal, just to go up on stage and find out a bandmate, or bandmates, completely fall apart on stage. Personally, I do agree with him, although I know a lot of people probably wouldn't, or don't.
Now, how you do that is another thing entirely. Should you have enough music? Absolutely. Is it a mortal sin to repeat songs? Nope. You certainly don't want to show up for a 4 hour gig with 12 songs, but spooning in a few repeats isn't the end of the world, especially if you play certain ones killer and they are good songs, and you can preface it with something like "We got a request to play this one again.....We're glad you liked it so much, so here you go!". It happens. Bands do it. A lot of times if it's been a few sets since you last played it, you'll get away with it. Do you want to make a habit of it? Probably not. It should be a temporary crutch if you use it at all. For some bands it's tougher because they don't have people in and out during a show....like our band, typically we're blessed (or cursed) enough that people just don't leave until the end. If you really don't have enough music to fill 4 hours, at least reasonably, you probably should take a shorter job or wait until you think you do have enough. Maybe better to see if you can open for a band, play an hour or so, see what you've got that way. Or maybe not. Maybe just jump right in and see if you sink or swim, especially if you're "really, really close". You gotta take opportunities when they come up, because for a new band you just don't know when that might be, the one place that is willing to take that chance, and give you your first gig (presumably paid one).
These discussions are always good, and you can get an idea of how other people do it or how they view it, but in the end they don't establish a solid answer because everyone has their way of doing it, and each person is different. Each situation is different.