When the prefix is used to express something is repeated, it is always re like:

• prendre -> reprendre
• paraître -> reparaître

except if the verb starts with a vowel in which case it is either ré:

• agir -> réagir
• essayer -> réessayer

• élire -> réélire

• inventer -> réinventer

or r with elision:

• emballer -> remballer
• ouvrir -> rouvrir
• emporter -> remporter
• échapper -> réchapper
• épandre -> répandre

There is no rule to know if the elision is to be made or not. In particular a very common mistake is to use réouvrir instead of rouvrir due to the proximity of réouverture.

Otherwise, when the verb happen to start with re for other reasons, both forms can be found, depending on the verb root/etymology.

• recroqueviller
• recruter
• refuser
• regretter
• relater
• relativiser
• relaxer
• remorquer
• régner
• réguler
• répéter
• répondre
• révéler

The verb réparer is kind of an exception as etymologically, it cames from the Latin re-parare.

Other exceptions found in espacefrancais.com/laccentuation-de-la-voyelle-e are:

• concilier -> réconcilier
• chauffer -> réchauffer