environmentVar could use a "unique" constraints

[jkowalleck]

          since a var can only have exactly one value, it is desireable to have a 

constraint that each var is used only once in the context of it's environmentVars container.

Originally posted by @jkowalleck in #222 (comment)

[jkowalleck]

I'd suggest to drop this from 1.6
this looks like a breaking change !