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Q. Can you give another Bouquet to a Hero whom you have already given a Bouquet to?
You can.
However, the previous Bouquet's effect will be lost, and the new Bouquet's effect will be reflected.
Q. When you give a different Bouquet to a Hero what happens to their Support Rank?
Their Support Rank will carry over as is.
For example, if they were S+, then they will remain S+.
Please take a look at the below Learn With Sharena article about Bouquets and Entwined Heroes.
【Enhanced Ally/Summoner Support with Entwined Heroes!】