[basic.def.odr]p6 says:

"If the definitions of D satisfy all these requirements, then the program shall behave as if there were a single definition of D."

However, this is obviously a constraint on the implementation, not on the program, so the use of "shall" here is incorrect. This should instead say:

"[...] then the program behaves as if there were a single definition [...]"

or

"[...] then the implementation shall behave as if there were a single definition [...]"

While I think the intent is clear here, I'm not entirely sure this is an editorial change; if not, let me know and I'll take it to CWG.