`Fbe::Conclude#follow` silently overwrites on second call, unlike `by` and `as` which raise `Fbe::Error`

[morphqdd]

What happens: In lib/fbe/conclude.rb, calling follow a second time silently replaces the previously configured list of properties to copy, with no error raised. The by method guards against double-invocation with raise(Fbe::Error, 'Query is already set') unless @query.nil?, and as does the same with raise(Fbe::Error, 'Label is already set') unless @label.nil?. The follow method has no such guard.

What should happen: follow should raise Fbe::Error when @follows is already non-empty (i.e. when called a second time), consistent with the defensive pattern used by by and as.

[morphqdd]

@yegor256, please take a look when you have a moment.

[0crat]

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