Check for duplicate constraints in `addconstraint!()`

[pwochner]

Currently, addconstraint!() simply pushes a constraint to the grammar.constraints vector, without checking if the constraint already exists.
We only want to add a constraint if it doesn't exist yet.

[ReubenJ]

Not exactly related to Herb-AI/HerbConstraints.jl#81, but somewhat related. Both fall under the umbrella of defining equivalences for T<:AbstractConstraints.