make -jN runs some rules more than once

[Brian Malehorn]

Consider the following Makefile:

all: a.gz b.gz

a.gz b.gz:
        touch a
        gzip a
        touch b
        gzip b

If you run it with 'make -j1', it runs fine.

But if you run it with 'make -j2',
  - target "a.gz" must be built, spawning job 1 to build "a.gz b.gz"
  - target "b.gz" must be built, spawning job 2 to build "a.gz b.gz"
  - they trip over each other and the build crashes

There are plenty of workarounds, but this strikes me as a bizarre
default behavior. Shouldn't the rule "a.gz b.gz" should only be run once,
regardless of -jN?

This is how 'make -j2' should have gone:
  - target "a.gz" must be built, spawning job 1 to build "a.gz b.gz"
  - target "b.gz" must be built, but the rule to build it, "a.gz b.gz",
    is already being executed. No job is spawned.

I ask because I just spent a few hours debugging a Makefile with the
false assumption that "a.gz b.gz" would only be run once. It's intuitive
that each each target should be built with a "task queue" rather than a
"parallel free-for-all", and I'm sure this design decision has lead to
pain for a lot of other programmers, too.

Thanks,
Brian