conversion error on multiple occurences of a non-vector option

[lambdafu]

If I use a std::string as an option (say, "-o") and the option appears multiple times ("-o foo -o bar"), there is a conversion error thrown, because the lambda returns false (it checks if size of result is 1).
Some observations:

• The exception messages are not very user friendly. They are too generic, not localized, etc.
• The exception type is the same for all options, so it's not easy to customize the message.
• Due to the simplicity of many getopt-based implementations, usually tools take the last-writer-wins strategy when options are repeated (for example, "cp -S .bak -S .orig" will choose ".orig" for backup copies).
• It is possible to implement such a strategy with a custom lambda function (that's nice!), but it's extra work and not immediately obvious.
• Another strategy is to always use vectors, and do the error handling / value folding post-parsing (barf!).

Simple things should be simple, and if only one "-x foo" should be allowed, then giving a diagnostics like "only one -x option allowed" should be made simple by the library. It doesn't need to be automatic, it's sufficient if I can catch the error and output an appropriate message.
In case that's not easy to do, maybe the last-writer-wins strategy for non-vectors is the better default strategy, because it can not fail by definition.
CLI11 shows some excellent design taste, and I don't want to blemish it with a specific proposal how to fix this. The main point of this ticket is to share one of the few frustrations I have with the library as a new user. Thanks!