dtlib: error out on duplicate node names

Attempts to define two nodes with the same name within a single set of
curly brackets should fail.

For example, this is invalid DTS according to dtc:

    / { foo {}; foo {}; };

By contrast, this is valid since the node named 'foo' appears twice in
two different sets of curly brackets:

    / { foo {}; };
    / { foo {}; };

Zephyr's dtlib currently does not error out on the invalid condition.

Now that Zephyr itself has been updated to not include such nodes (to
the best of my ability), we can fix this divergence from current dtc
behavior and add a regression test in dtlib.

Fixes: #49590
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>