I have created my form and can add to my table, how would i write an if statement or other constraint to check the the data is unique, for example I can only have one course date for one venue?
You might handle this initially in the table, not the form. It would be handled by having a unique composite index for these two fields which would prevent duplicates being added to the table. Having done this you may still have some code to check for duplicates before inserting but this really depends on how you want your form to work - for example if the user enters a venue first, you could give them a list of available dates - i.e. excluding those that are not available.
And it also depends on how your table works - if someone has booked a course for 3 days, do you create a record for each day or one record with a startdate/enddate?