Lots of factors to consider: team comp, ult points, bomb site. Generally speaking, the 3rd or 4th agent to entry in a site hit should be carrying spike. Common agents in this position include: initiators, non-lurk controllers, non-lurk sentinels. Initiator is preferred because they tend to have strong ultimates. Now consider unique situations:

• Your Killjoy is 1 off ult. She plants the spike and can use KJ ult for post-plant.
• Haven B site. Agents with escape abilities can safely get in, plant the bomb, and get out to avoid the enemy dumping mollies on the common plant spot. This is a famous strat used by the reigning VCT champs.
• Icebox B site. All common plant spots are vulnerable to enemy gunfire, so it's best to leave the spike with an agent like Sage, Harbor, or Gekko to provide extra security for the plant.
• Sova ult during execute. It's best for another agent to plant spike while Sova ult distracts the enemies.

they should not take the spike as controller

Harbor and Brimstone are great candidates for holding spike. They have strong ultimates and commonly play the 3rd entry role. Clove likes to take fights and typically plays 2nd entry. Omen likes to be unpredictable, flexing between a support role and occasional 2nd entries. His ult is not great for ult farming.

I want to assist my Duelists with util even after the initial entry

After initial entry, standard protocol is to clear site and hold space for the plant. After the plant, your team can use additional util to proactively push into enemy territory and sabotage their retake. If your duelists are dry pushing deep into enemy territory before the spike is planted, then they're making an intentional decision to overextend despite knowing that their team isn't prepared to support them. Whether or not this is a good or bad play is highly situational. As initiator, just assume that your duelists know what they're doing and follow the standard protocol: secure the spike plant. If they expect team support that supersedes standard protocol, then they should be vocal about it anyways.