Do you have rules set for your sequence? Go to the settings for your sequence and exclude based on tags or if they're in another sequence!

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Do you have rules set for your sequence? Go to the settings for your sequence and exclude based on tags or if they're in another sequence!
Another option in addition to @Emily Mills52 great suggestion is to use a tag in the Visual Automation to prevent subscribers from getting added to a welcome sequence if they are already getting one.
If you have one automation per welcome sequence, then you can add a tag as soon as someone enters the automation.
Then in the second (and other) welcome sequences, create a condition to check if the subscriber has that tag. Only add them to the sequence if they don't have it.
And finally, at the end of the automation, remove this tag.
Here is an example: