Fix bug 28600 by doing INSERT IGNORE, just like LinksUpdate does.
Fix bug 28600 by doing INSERT IGNORE, just like LinksUpdate does.

WouterRademaker/mediawiki-extensions-GlobalUsage,wikimedia/mediawiki-extensions-GlobalUsage,wikimedia/mediawiki-extensions-GlobalUsage,WouterRademaker/mediawiki-extensions-GlobalUsage