The easiest way I have found is to go to "View - Places - By all places".
Scroll to the top of the list and then down, looking for duplicate places.
When you find a duplicate, double click on it and a screen will come up for that place.
Click the "Find" radio button and if you are lucky, the place will be displayed. If not enter the place name in the "Find again" block anc click the "Find again radio button.
Select the place you want to keep as the suvivor and click the "Identify" radio button. Click "OK" and select the duplicate place that you wish to merge with the surviving place and click the "Merge" radio button twice. (Once to return to the screen and once to select the function)
A box will appear and if you agree click "OK" otherwise click "Cancel"