Option cannot be specified more than once

[martinpopel]

In #1082 (comment) @arielf wrote

Seeing how many users get hit by the "cannot be specified more than once" error.
I'm thinking maybe we should rethink it.

I think this should be discussed in a separate issue (here).
I like @arielf 's suggestion

• Check and if the two 'specified' values for the option are the same, don't abort

[martinpopel]

One special example to consider: option --probabilities does not change the way a model is trained, but it could be useful in training because it reports multi-class logistic loss (in addition to zero-one loss). The main purpose of --probabilities is for testing to convert the predicted values (-p) to probabilities.

There may be several models, some trained with --probabilities, some without and I may want to test all of them and predict probabilities. Currently, it is not possible (I need to provide the --probabilities option only for those models which were trained without it).

For me this is a low-priority issue - I have not encountered the problem above yet. But for newcomers, the error may be counter-intuitive (it says "more than once", but they see the option just once on the command line, they are not aware that it is saved in the model).

[JohnLangford]

This is fixed in the master now.