To Avoid Duplicate Messaging, Make Sure Your Contacts Are Only in the Appropriate Sequence(s)

If you attempt to add contacts to a sequence when they are already in one (or more), a warning modal will ask: "Are you sure you want to also sequence the following contacts?" 

When this happens, you can either: 

1. Leave the checkbox unchecked and then click Sequence Remaining Contacts. This will ensure the noted contacts are NOT added to the sequence.
2. Check the box next to the list of contacts active in other sequences and then click Sequence Remaining Contacts. This will cause the noted contacts to be added to the second sequence

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This warning is intended to prevent you from adding contacts to a sequence when they are already in one, just in case that's something you'd like to avoid. That said, sometimes it will be necessary to add the same contacts to multiple sequences.