Why is transferred written with two R's? I am a native speaker of Dutch, and in my point of view this isn't logical; there are other words like coloured and endeavoured that only have -ed added after the verb.
The verb was borrowed into late Middle English as transferren, either from Old French transfer(r)er or directly from Latin transferre. It was stressed on the second syllable, as it is for many speakers today. Verbs ending in stressed [ɜ:] (non-rhotic varieties) or [ɝ] (rhotic varieties) typically double the final r in forming the past tense and the participles: occur ~ occurred, infer ~ inferred. Transferred is simply following the same rule.
The complete rules for doubling final consonants in this context are complicated and riddled with exceptions; I’ve mentioned only the one that is most relevant to this specific question. In general Middle English stress and vowel length are the most important factors, with doubling tending to occur after stressed short vowels but not after long ones. Of course not all verbs go back to Middle English; those that don’t are generally handled by analogy with those that do.