If I have an equipment attached to a unit, can it still be the target when playing Relentless Pursuit even if the unit targeted is the same one with the equipment attached?

Yes, you can play Relentless Pursuit targeting a unit that already has an equipment attached, and you may choose to attach another equipment to that same unit.

Reasoning and Rules Clarification

1. Multiple Attachments: According to Rule 744.3.b, "A Top-Most Card is Equipped as long as one or more of its Attached cards are Equipment." There is no rule in Riftbound that limits a unit to a single piece of equipment. Attaching a new piece of equipment does not cause existing equipment to detach; the unit simply becomes equipped by both.
2. Targeting: When you play Relentless Pursuit, you must choose both a friendly unit and an equipment you control. The spell text "You may attach an Equipment... to it" is an optional instruction.
3. Interaction: Because the unit is a valid target for the move effect, the spell can be played regardless of whether the unit is already equipped. During resolution, you may choose to attach the chosen equipment to that unit, even if it already has one.

Summary of the Interaction

• Playing the Spell: You target a friendly unit and an equipment you control.
• Resolution: The unit moves. You then have the option to attach the chosen equipment to that unit.
• Result: If you choose to attach the equipment, it is added to the unit. The unit now has multiple pieces of equipment attached, and both will continue to provide their effects (as per Rule 421.1.c, which states the Top-Most card has all effect text of attached cards appended to its rules text).

Note: I could not fully verify this ruling against the retrieved rules.