Cargo easyblock produces confusing error message when same crate is listed twice

[boegel]

When humantime 2.1.0 is listed twice in crates, then the Cargo easyblock produces a very confusing error:

ERROR ... Unpacking sources of 'humantime-2.1.0.tar.gz' failed

It's actually failing to detect which subdirectories get created by unpacking humantime-2.1.0.tar.gz the second time.

So, Cargo should check whether a particular crate is listed twice, print a warning, and ignore it after unpacking it for the first time?

[akesandgren]

We should also make the easyblock verify the type of the crates variable. If it's not a list one will get weird errors.