Please post your image (or a lores version).
One can create Patterns in Photoshop, but an »overlap« as in the example may take a little effort.
Do you have Illustrator? If so I would advise you to use it to create a repeat pattern such as this.
Here is a good tutorial: How to Create Seamless Patterns in Illustrator - YouTube
I hope this helps!
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Here is a tutorial on how to create a repeated pattern in photoshop. How to create a seamless pattern in Photoshop CC - YouTube
But if you are creating this for a t-shirt I would more likely do what Simmer1 said to do in illustrator.
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Depending upon your design, you may find the result you want this way:
Open the file and reduce its size.
Choose Edit > Define Pattern
Open a new blank file 8 x10 or larger
Choose Edit > Fill > Pattern. Yours is the last in the set.
Experiment with various Scripts and sliders within them.