Duplicate names

[blegat]

We want to stop requiring models to error when a duplicate name is set, see #535.
What should be the requirement now ?
Maybe we can require it to error when the user does MOI.get(model, MOI.VariableIndex, name) where two or more variables have the name name and if there is only one variable having the name name but some other names have duplicates then it might error or not, we leave the choice in the implementation (in model, it errors when building the dict: #535)
What do you think ?

We should also decide whether a model is allowed to error on MOI.set even if it is not required

[mlubin]

We could say that it is "invalid" for any two variables or two constraints to have the same nonempty name and leave it up to the implementation on how to detect it and when to throw an error. Implementations could choose to throw an error on the set or on the get or at some other point. What we should write down is that there must be an error either on the get or before. To catch user errors, implementations aren't allowed to silently ignore the existence duplicate names like Gurobi does in their interface.

We can't write generic tests for this behavior; models would have to write their own tests to make sure they're consistent with the rules. It's not ideal, but we're constrained by the performance considerations.

[blegat]

We can't write generic tests for this behavior

Why can't we write the test

@test_throws ErrorException begin
    MOI.set(model, MOI.VariableName(), x, "a")
    MOI.set(model, MOI.VariableName(), y, "a")
    MOI.get(model, MOI.VariableIndex(), "a")
end

solvers can throw an error in either the second or third line.

[mlubin]

@test_throws ErrorException begin
    MOI.set(model, MOI.VariableName(), x, "a")
    MOI.set(model, MOI.VariableName(), y, "a")
    MOI.get(model, MOI.VariableIndex(), "a")
end

We can try that.

[blegat]

Note for future reference, we have changed our mind and setting duplicate names is now valid. Only get throws an error, see #548