[kazinator]

There are regional variations in the use of doubled or single consontants:

UK: travelling, signalling

USA: traveling, signaling

The double l serves no purpose in these.

However, there is no such UK/USA split in "referring": both use double "r".

There is a phonetic reason for that which is that the second syllable has the emphasis in "refer", unlike "traveling" and "signaling" where the leading syllable has it.

The double "rr" indicates the stressed syllable, without which "refering" and "referer" could be misread with emphasis on the first syllable, like "buffering" or "suffering"/"sufferer".

A native speaker with good intuition for spelling would never write "refering" or "referrer", even if they find "signaling" acceptable.