dabbrev-case-distinction is a variable defined in `dabbrev.el'.
Its value is

Whether dabbrev treats expansions as the same if they differ in case.

A value of nil means treat them as different.
A value of `case-replace' means distinguish them if `case-replace' is nil.
Any other non-nil value means to treat them as the same.

This variable has an effect only when the value of
`dabbrev-case-fold-search' specifies to ignore case.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.