Suppose you have a schema with the following structure:

myfields
    ns1:coolDocument
        coolField
    ns2:coolDocument
        coolField

When you save your form to SharePoint, if you select to promote both coolField entries, then the promotion will fail for the 2nd entry (actually, it will read the first entry instead of the second).

The reason this happens is because SharePoint XML parser doesn't support namespaces, so the pseudo xpaths are actually just /myFields/coolDocument/coolField, i.e. there's no ns1: prefix. That means that the second column will extract the value from the first coolField.

More details: the SharePoint XML parser completely ignores the namespaces and has no support for XPath. This is why the XFP uses the name “node“ for the attribute versus XPath. It is not really an XPath but rather a list of element names separated by slashes.

WORKAROUND:

Don't use or promote elements with the same name if they have different namespaces.