Duplicate header deps in gcc depfiles

[evmar]

I noticed multiple instances of the same header in a local depfile. I haven't yet tracked down why this occurs. See also #418.

[Lekensteyn]

This occurs whenever the deps command generates duplicate dependencies. Can we just require non-duplicate entries from the command for simplicity?

build.ninja

rule gen
  command = echo "$out: x.h x.h" > $out.d && touch $out
  deps = gcc
  depfile = $out.d
build x: gen

Output of ninja -t deps

x: #deps 2, deps mtime 1508637043 (VALID)
    x.h
    x.h

[mathstuf]

Couldn't the input list be deduped when reading as well (with a warning available for it possibly?). Though I think the logic pending #1534 seems to dedupe anyways. I now get this:

x: #deps 1, deps mtime 1580311346270505630 (VALID)
    x.h

[mathstuf]

@jhasse Seems like this can be closed as well.