I was traveling to Dubai on a tourist visa. I was stopped by Immigration and my visa was cancelled; they also wrote CWOP on the stamp.
Will this cause problems the next time I travel?
It means the visa was canceled, but cancellation does not affect future applications and issuance of visas.
From the US State Department Glossary definition (thanks to @richardb):
Cancelled Without Prejudice: A stamp a U.S. Embassy or Consulate puts on a visa when there is a mistake in the visa or the visa is a duplicate visa (two of the same kind). It does not affect the validity of other visas in the passport. It does not mean that the passport holder will not get another visa.
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