Magento 2.1: setup:di:compile error Constant MASK_FOR_HIDDEN_FIELDS already defined

When we run the setup:di:compile command we are seeing the following error. I would like to know what is going on here before I contact the extension developer.

I have searched for “MASK_FOR_HIDDEN” in all files and it only exists in the file in the below error and also in one other file. Is this the problem? Do they need to be unique in both files?

[Exception] Notice: Constant MASK_FOR_HIDDEN_FIELDS already defined in /home/signedge/p ublic_html/idcardsolutions.net/app/code/Magenest/SagePay/lib/classes/Sagepa yUtil.php on line 4

Thanks