Is the duplicate content issue a myth? I have heard that duplicate content does not affect you unless you have the same content on the same domain. Otherwise if it's on a different IP address it's known as Syndication. However, I have done a Google search of a few of my article paragraphs in "quotes" and I have found that sometimes the articles that come up on the first result that Google considers to be first importance is not my site! I know my articles are 100% unique, but then I use AMA and UAW and spin 3 versions of the same article and get it syndicated.

I have been able to achieve first place rankings for many keywords on this on Google, however I'm worried that some day this might be detrimental to me?

Duplicate content is definitely not a myth. There's waaay more to this subject than I can even spell out here in a 500 page thread.

"A similarity engine generates compact representations of objects called sketches. Sketches of different objects can be compared to determine the similarity between the two objects. The sketch for an object may be generated by creating a vector corresponding to the object, where each coordinate of the vector is associated with a corresponding weight. The weight associated with each coordinate in the vector is multiplied by a predetermined hashing vector to generate a product vector, and the product vectors are summed. The similarity engine may then generate a compact representation of the object based on the summed product vector."

Even spun articles can be considered duplicate content... If you were doing this though, at LEAST spin to 50%, ignore the 30% generally recommended. Someone once told me that any 2 articles in the english language can be a maximum of 70-80% unique, regardless of the particular topic they're used on (due to speech patterns, sentence structure, etc).