Error: redefinition of should show where previous definition is; in some cases can be super confusing

[timotheecour]

see example here: #8580, which I'm rewriting here:

#[
BUG:
/Users/timothee/git_clone/nim/timn/bugs/stdlib/t01_mapIt_redefinition_of_t.nim(6, 1) template/generic instantiation from here
/Users/timothee/git_clone/nim/timn/bugs/stdlib/t01_mapIt_redefinition_of_t.nim(9, 9) Error: redefinition of 't'
D20180808T142452
]#

import sequtils

when defined(case1):
  proc test() =
    # BUG
    let t = [1,2].mapIt(it)

test()

In this case it's hard to spot for users not familiar with implementation of mapIt where the previous definition was.
There are similar cases

the compiler knows where previous definition was, and should just show it

[Araq]

Duplicate of #447. It's somewhat hard to implement as it requires minor compiler refactorings.

[timotheecour]

redefinition error messages were improved in 539fc5d