IP transfer question

If I manufacture a product under license from its designer, I unavoidably
learn aspects of its design which I may then, intentionally or not, use in
my own subsequent designs.

Suppose that over time, I evolve the product's design, perhaps to the point
that it is no longer recognizably derivative. Typically, how is the
question of "when do I stop paying royalties" addressed in contracts?

That is, what contractual terms are generally used to protect the other guy
from me just tweaking his design, calling it my own, and selling it without
paying royalties; and, on the other hand, to protect me from him claiming
that any vaguely-related product I ever subsequently design is really his
and I owe royalties on it?



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