Why were prefixes repeated as postverbal prepositions?

Question

French: s'abstenir de    Spanish: abstenerse de    English: abstain [from] (v.)
late 14c., "to withhold oneself,"
from Old French abstenir (14c.), earlier astenir (13c.) "hold (oneself) back, refrain, abstain (from), practice abstinence,"
from Latin abstinere "withhold, keep back, keep off,"
from ab(s)- "from, away from" (see ab-) + tenere "to hold" (see tenet).

French: adhérer à    Spanish: adherir a    English: adhere to (v.)
directly from Latin adhaerentem (nominative adhaerens), present participle of adhaerere "stick to,"
from ad- "to" (see ad-) + haerere "to stick" (see hesitation).

French: appartenir à   English: appertain [to] (v.)
late 14c., from Anglo-French apartenir, Old French apartenir (12c.) "be related to; be incumbent upon,"
from Late Latin appertinere "to pertain to,"
from ad- "to, completely" (see ad-) + pertinere "to belong to" (see pertain).

1. Literate Anglophones and Francophones were fluent in Latin, and comprehended the Latin prefixes, when these verbs were borrowed from Latin. So why did English and French repeat the Latin prefix as a postverbal preposition?

2. To wit, couldn't intransitive verbs above be ditransitive?

For the following cognates, the English cognate appears more redundant than the French and Spanish ones. English added the prefix en- , but 'ENdow' must still be followed by 'WITH'. French didn't add any prefixes and conveys en- with 'de'.

French : doter qqn de qch (= endow somebody WITH something) 
Spanish: dotar de algo a
English: endow (v.) [...] from en- "in" (see en- (1)) + Old French douer "endow,"
from Latin dotare "bestow" (see dowry).

Answer 1

Literate Anglophones and Francophones were fluent in Latin, and comprehended the Latin prefixes, when these verbs were borrowed from Latin. So why did English and French repeat the Latin prefix as a postverbal preposition?

The same was done regularly in Latin as well! One can say in X ineo or ex X exeo. This kind of repetition is quite typical, and communication is often improved by redundancy.

It may also well be that a prefixed verb becomes detached from its original compound meaning and becomes a more independent verb. This is particularly natural for borrowed verbs; the English "adhere to" works best with an English preposition "to". Although some will understand that ad- means "to", most will not.

To wit, couldn't intransitive verbs above be ditransitive?

Perhaps they could, but they aren't. Languages don't always evolve in the most logical way.