plagiarism:

I am really researching copy writing and getting a feel for whether I think its for me. I have this question in my head that need to be set it free before it starves and dies.

I live on a constant diet of Comedy radio and similar media. Are there time's when you might think you have a original idea? To find out later that it was something you heard someone else do. Like having a tune stuck in your head.

Do you get a mulligan or a pass for an error like this or does it turn into a big/ bad?

You always have to be careful with the things that you write as a copywriter, it's like with anything else. Having said that, however, we live in a day and age where it is extremely difficult to come up with something original and things change from day to day so quickly that you never know whether someone on the opposite side of the world from you is doing exactly what you're doing. Also, if you have a look at most of today's advertising from category to category, you will find that it's all looking relatively generic. The cosmetics or car industry tend to have very similar (if not the same) types of advertisements with a couple of the words jumbled around.

My advice would be to do the best that you can to check whether something that you have come up with has been done before. Start with Google-ing your idea and go from there. The best way (in my opinion) to approach any project that you do as a copywriter, is to first start off with the research - that way you know you've covered at least a half of the things that have already been done.

Good luck!