Sequence - error option is defined multiple times

[Wiki - Sequence option](https://github.com/commandlineparser/commandline/wiki/CommandLine-Grammar#sequence-option) states:

> Sequence option
Sequence options require one or more argument value.
its type is: IEnumerable
Option name may be repeated more than one time.
Thus --files file1.txt file2.txt and --files file1.txt --files file2.txt are equivalent.

In fact, using the second example by specifying the parameter twice leads to an error:

ERROR(S):
Option 'input' is defined multiple times.

Start with the repeated-option examples in the issue and compare their behavior with the Sequence option rules in the linked wiki page. No source files or tests are named; done means repeated sequence options are accepted as equivalent to one option with multiple values, without the duplicate-definition error.

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