Allow diamond-style includes

[krlmlr]

Multiple includes of the same .yml currently gives All target names must be unique, this seems unnecessary. Example:

# base.yml
targets:
  test:
    command: quote(TRUE)

# a.yml
include:
  - base.yml

# b.yml
include:
  - base.yml

# remake.yml
include:
  - a.yml
  - b.yml

[krlmlr]

We should also consider making recursive includes a no-op (with some output).