Is it permissible to repeat ablution (wudu') when one is already in a state of valid ablution, and does this act result in greater reward, or is it disliked (makrooh)?

It is sunnah (recommended) to renew one's ablution (wudu') even if one is already in a state of purity, according to the majority of scholars. This is based on what was narrated from Anas ibn Malik (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) used to perform ablution for every prayer.

Renewal of ablution is recommended if a Muslim has prayed a fard (obligatory) or nafl (voluntary) prayer with the first ablution. However, if no prayer has been performed with it, then renewing it is not prescribed.