Equal and opposite effect

In 2000 I wrote, published and claimed copyright on this concept (bolding added now):

This is a very interesting principle, a concept that I have created. One that upon mastering, will assist you to avoid negative outcomes from training. The concept is based on the belief that to every action (in training) there is a positive and a negative outcome, and that often the negative outcome is equal or as powerful as the positive outcome.

I was conducting research and I came upon the following exercise in a publication copyright claimed by another 'author' at a subsequent date (bolding added):

The concept is based on the belief that to every action (in training) there is both a positive and negative outcome, and often, the negative outcome is equal or as powerful as the positive one.

And I said to myself: “That looks familiar!” So I cross-referenced it and I said to myself: “Wow! No wonder that looked familiar!”

This was different! The 'author' gave credit to Ian King for the concept. But failed to place the subsequent text in parentheses...

And I came upon this wording another one more times by the same 'author' in different publications.