cp: normalize paths before checking for duplicate sources

[matrixhead]

The cp command generally does not allow duplicate source files.
for example

touch a; mkdir d;
cp a a d/ 

This produces:

cp: warning: source file 'a' specified more than once

Additionally, when providing paths like this:

cp ./a a d/ 

GNU cp correctly identifies these two paths as pointing to the same file, while our implementation currently attempts to copy both paths, resulting in:

cp: will not overwrite just-created 'd/a' with 'a'