The way I see it, if your band is going to play a cover song that the earlier band played, you have to make sure that your cover is going to be different, or atleast it introduces a new way of 'appreciation' interms of the audience's point of view. This actually happened to us during a recent gig (Music Ring). Never Ending Problems covered My Heart (they did it beautifully), but since our interpretation of My Heart was different from theirs, we felt confident enough to play it.
Either you do that, or have backup songs. Most of the time you get to play 5songs on a 30minute set, just make sure that you guys are confident enough to play 7songs incase anything happens. Although I know sometimes bands plan what they're going to play and how they're going to transition from one song to the next, so it'll really mess up the whole show if you change the song list at the last minute, so I guess you just have to hope and pray that this never happens to you.
