Splitting <div contenteditable="true"><span id="x">ab</span></div> between "a" and "b" results in <div><span id="x">a</span></div><div><span id="x">b</span></div>.

This is related to bug 1449874.

There is code (in HTMLEditRules.cpp:SplitParagraph) to avoid the duplicate id when splitting a paragraph tag (e.g. when splitting <div id="x">ab</div>), but it is insufficient to handle this case.

This minimal patch seems to fix the example.