I think Fredo would have done it again. He wanted to get Michael out of the way. Without Michael he might feel a sort of power. He might even feel respected. I think Fredo wants the power he couldn't receive and that drove him crazy. And he told Michael that that reason he did what he did was because there was something in there "just for him." Michael saw Fredo as unloyal so he just had to get rid of Fredo.

If Fredo of had done it once for that reason, he would definitely do it again.