Under normal circumstances it's fine to require the same file twice, because require keeps a record of the libraries it has loaded and ignores a second call.
It looks as if you get this error if the library you are requiring requires itself somehow, which is not something that require can cope with -- the require operation presumably needs to complete before the library is added to the list.
So if in a.rb you require b.rb and in b.rb you require a.rb. then when you write `require "a"` Ruby will
* start to require a.rb
* start to require b.rb
* start to require a.rb again and realise there is something terribly wrong.
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