When reading this, one could think that executing the same define-key
statement multiple times makes the KEYMAP grow. However, this is not
the case. Glancing at the code, I see that it seems to alter the old
binding, if one is already present. So one possibility to change it
is like this:
If KEYMAP is a sparse keymap with a binding for KEY, the existing
binding is altered. If there is no binding for KEY, the new pair
binding KEY to DEF is added at the front of KEYMAP.
The wording is not very good, I think.