[ruby-core:103566] [Ruby master Bug#17821] Impossible to define only short versions of options in OptionParser

Issue #17821 has been reported by fidalgo (Paulo Fidalgo).

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Bug #17821: Impossible to define only short versions of options in OptionParser
https://bugs.ruby-lang.org/issues/17821

* Author: fidalgo (Paulo Fidalgo)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x86_64-linux]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
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I have tried several approaches and either I'm being misled by the documentation or there's a bug.
I've tried this with two versions of ruby 2.7.2 and 3.0.1, both compiled using rbenv.

Here is the code:

```
require 'optparse'
options = {}
OptionParser.new do |opts|
  opts.banner = "Usage: #{$PROGRAM_NAME} [options]"

  opts.on('-na', 'Numbers for the first column') { |o| options[:na] = o }
  opts.on('-sa', 'Stars for the first column') { |o| options[:sa] = o }

  opts.on('-nb', 'Numbers for the first column') { |o| options[:nb] = o }
  opts.on('-sb', 'Stars for the first column') { |o| options[:sb] = o }

  if ARGV.empty?
    puts opts.help
    exit 1
  end
end.parse!(into: options)

p options
```

and the output:

```
ruby options.rb 
Usage: options.rb [options]
    -nb                              Numbers for the first column
    -sb                              Stars for the first column
```

The bug happens because we should have four different options on the help.



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