Even though back in 2013 it wasn't possible to easily suggest merging products, now it is:
Go to the Contact us page, select Products and inventory → Product page issue, then Merge or split product page:
After you've reported a duplicate, you can check on the progress at https://sellercentral.amazon.com/gp/case-dashboard/lobby.html
Alas, the merge isn't going to happen quickly if the products have different UPCs (which apparently sellers ensure is the case by buying different UPCs). Here is the reply I got after asking to merge three listings for the same LG Infinim Bluetooth headset:
Thank you for writing to Amazon.com. I understand that you want us to merge the ASINs B00NW2JBYO, B00Q5D0LUU, B00QQYZS0Q in our catalog. I will surely assist you in this regard.
I have viewed your request and upon investigation I noticed that the UPC of the ASIN does not match as per our catalog records. Kindly note that in cases where the UPC does not match, we at seller support do not have the authority to process the request from our end.
However, to make sure our catalog is error free and updated, I am proactively forwarding this issue to our catalog department internally, who have the authority to take decisions in this regard and in such cases. Kindly note that this is done through a systematic process that requires review and research of the duplicate items, which can take time to resolve. Once the duplicated items are confirmed, they will be merged into a single ASIN by the concerned team. I request your understanding and co-operation in this regard.
I have to inform you that we would not be able to update you on the status of the request in this regard. However, as soon as the ASINs are merged, all the sellers will receive an email informing about the merge and the reason as to why it was merged.
I sincerely apologize for any inconvenience caused. Kindly write back to us in case you need any further assistance in this regard and I will be glad to assist you.