Go into Flat View in the folder, then Select All, then click this button:
(See here for what to do with the button XML.)
<?xml version="1.0"?>
<button backcol="none" display="both" textcol="none">
<label>Select Duplicate JPEGs</label>
<icon1>#default:selectwild</icon1>
<function type="normal">
<instruction>@nodeselect</instruction>
<instruction>@nofilenamequoting</instruction>
<instruction>Select PATTERN="*.jpg" DESELECT</instruction>
<instruction>Select PATTERN="{file|noext}.*"</instruction>
<instruction>Select PATTERN="~(*.jpg)" DESELECT</instruction>
</function>
</button>
You should then be left with just the .JPG files which have the same name as other files.
(It doesn't check that the other files are .DNG, so if that's important the button will need to be modified slightly.)