First you need to make sure the contact(s) you want to re-engage are not active in any sequence. Then, check the following steps
Go on the Sequences page (1) using the left side pannel
Click on the sequence in which you want to re-engage contact(s) (2)
Go up in the Parameters tab (3)
Go on the toggle button Allow contacts that were previously engaged in another sequence to be added in this one (4). Switch it from grey to blue by clicking on it
To finish click on the Save the sequence button on the right (5)
Get your contact(s) and add them to the sequence (feel free to check this article to know more about adding contacts to sequences)
Other method
You can directly activate this toggle button when adding contact(s) to a sequence.
Indeed, if any of the contacts you are about to engage was already in a sequence, you get the following message :
Click on this message, then on Authorize adding (6) then on Next (7):
Use case :
When editing your sequence, if you know that you are going to include contacts that you have already contacted in a previous sequence, we recommend you to enable this toggle (switch it in blue).
However, the toggle is a protection that prevents you from sending too many sequences to your contacts. This is why this safeguard is enabled by default to prevent any spam or deliverability issues.