init: should error on duplicate stanzas

Currently when init parses a conf file, if you have duplicates of any stanza (e.g. "start on" or "exec") it uses the last one and silently ignores any previous.

This was done on the basis it would allow an easy way to do "overrides", just parse one file followed by another. It was a bad idea.

Instead init should error on duplicates of any new stanza, or any duplicate of an old stanza in a conf file that contains any new stanza. (Not just error unconditionally because of backwards compat, but we could at least warn somewhere)