Help with workflow with PRE and equal to field value

I have a workflow that triggers "when modified record saved". I want it to work the following way.  When the user populates for the first time (or changes) a field to a specifc value (U.S or Canada), a task is created.  The workflow is creating the task fine when US is entered, except now each time the record is modified, because the value is filled in on the field, it creates another task.  I think I am just missing  something simple, but I cannot get it to only create the task the first time the field is populated.  Can anyone please