Resolution:
This is designed behavior if you referenced variables in the alert content. Please refer to TIBCO Hawk document -> TIBCO Hawk Administrator’s Guide -> Chapter 7 Advanced Rulebase Feature -> Reference Variables in a Rulebase -> How Variable Substitution Affects Actions for the detail info.
If you still want the alert be sent out once, there are two ways:
1.    Don’t use the variable in the alert content.
2.    Use the “Maximum of X Times, No sooner than every X seconds” feature. Please refer to TIBCO Hawk Administrator’s Guide -> Chapter 4 Monitoring with Rulebases -> Defining Actions on how to use it.