-Ycheck should ensure global uniqueness of definitions

[odersky]

#203 uncovered a problem where a closure was generated twice with the same symbol. It was caught by phase RestoreScopes when the closure symbol ended up in two definitions in the scope of the enclosing class. This would have been caught earlier if -Ycheck verified that every symbol is defined only once in a compilation unit. Right now it detects only double definition in the same scope, or nested scopes.

[DarkDimius]

f236282 implements this as warning.
It seems that there are quite a few of doubly defined symbols generated in dotty. Could be seen here: https://travis-ci.org/lampepfl/dotty/builds/39833020

[smarter]

f236282 implements this as warning.

So can we close this issue?