This article provides you with a step-by-step guide for allowing contacts to go through the journey multiple times

This will be useful in sending cart abandonment emails, post-purchase emails (reorder, recommendation, feedback, etc), or update profile information, etc., as often as the contact qualifies the criteria set in a journey.
Note - 

Please note that there is an upper cap that a contact can be added to the journey at the maximum 50 times in 30 days. 

Step1: In the journey builder, click on the Settings icon on the top right.

Step 2: Contacts entry to journey will be selected as Only once by default for all the journies. You can choose More than once and click Save for the contacts to be able to go through the journies multiple times.

Please enable the "Don't allow exist..." checkbox above if don't want the contacts currently present in the journey to be enrolled again

That's it. You're all set! Now the contacts will be enrolled in the journey as many times as the contact qualifies the criteria set in the journey