two AS announce the same adress space

What will break if two different ASes announce the exact same adress space? It's the old multihoming issue, small company wants to multihome, does not want to get an AS of its own, and wants to use one address space via both providers. This adress space is from PI space so ISP1 already announces the exact net (no aggregation) and would like ISP2 to also announce this, perhaps via a private peering using "private AS" but I would like to avoid that. I know the RFC says this should not be done (or at least I'm told so), but I would like to know if someone does this anyway and if they do, what problems arise? -- Mikael Abrahamsson email: swmike@swm.pp.se