eeschema erc: Duplicate hierarchical sheet labels do not cause ERC errors. (lp:#1853344)

Original report created by Victor W (vicw)

When running ERC on EEScheme, it is possible for child sheets to have
duplicate hierarchical sheet labels, and this does not trigger an ERC
warning or error.

Additionally, this results in those pins sharing a net.

I believe this is a bug because I can't think of a reasonable case in which
it is desirable to have duplicate hierarchical sheet labels that need to be
connected upstream; if you need to share a net, then the reasonable thing
to do is to have a single hierarchical label port (say, VCC), then use
internal net labels to connect them to several other ports.

However, if you don't realize that you have duplicate hierarchical labels
(as what may happen if you copy/paste hierarchical labels, and forget to
change one of them, but have different net labels), then you've
unintentionally caused a short in your design.

===
Example 1:

1. On the top level sheet of a hierarchical design, add a child sheet, with
some ports.

2. From within the child design, rename two of the ports to have the same
name, and synchronize it within the parent design.

4. Run eeschema DRC.

===

RESULT:

1. No errors or violations will be noted.

===

Expected:

1. I expect that duplicate hierarchical labels should trigger a warning;
it's not obvious that someone would do this intentionally. It's far more
likely that this was unintentional and should be flagged as a warning or
error.

Original tags: eeschema erc