Generally speaking, it's more a question of stress than etymology. If the stress is on the final syllable of the verb, the final consonant (r in this case) is doubled when ed is suffixed; if the stress is on a non-final syllable, the final consonant is not doubled.

(Incidentally, ferre with double r is the infinitive, and fero with one r is the 1st person singular present. Ferro would be the dative or ablative singular of ferrum, meaning iron and by extension anything made of iron, including swords)

However, moving away from verbs derived from ferre and looking in general at spelling changes when ed is added, there are exceptions to this rule: these are normally hesitations (you can see both focused and focussed for example) and spelling differences between British and American English (most notable travelled/traveled).