Duplicate entries in jac_structure

I have an issue when using `PDENLPModels.jl`.

When I call
```
(rows,cols) = jac_structure(nlp)
```
on some `PDENLPModel`, the tuple `(rows,cols)` can have duplicate entries which creates bugs on some linear algebra packages (like `QRMumps.jl`).
I think it would be easier to add a condition in the jac_structure function to prevent this.

I have a minimal working example if needed which is just the `test/problems/controleslaticmembrane.jl` example with `n=20`.