The "prevent duplicate assignments" option only prevents duplicates during the assignments on a particular time step. In other words, if you assign targets to particles A/B/C on frame 0, they won't have duplicate assignments. But if you assign targets to particle A on frame 0, particle B on frame 1, and particle C on frame 2, they can still have a duplicate assignment even with that option enabled because their assignment time steps are different (the duplicate checker only checks for duplicates within the group of particles currently being affected on the particular time step).
In your flow, your Time Test has frame variation, so they're not all moving to the next event on the same frame, hence the assignments are happening on different time steps.
A potential solution might be to assign on the same frame, then send out from that assignment event to the Find Target even with some variation.
In your flow, your Time Test has frame variation, so they're not all moving to the next event on the same frame, hence the assignments are happening on different time steps.
A potential solution might be to assign on the same frame, then send out from that assignment event to the Find Target even with some variation.