Upon entering, party triggered 2 notices and 2 separate HT preheads in channel 11 Cave of Trial I. After the party killed 1st prehead 1, I cleared the 2nd prehead 1. The party was able to proceed to prehead 2 as normal.
Not sure how much this helps but it clearly shows the spawn message twice. Out of all the times we went HT we have never seen this happen o_o