Single plainums are whats in from factory. They will Easily last 50k. Double platinums are far from necessary and tend to wear stupid in my experience with the spark sometimes just bypassing the platinum altogether. Gap the plug on the low side of the recommend spec to compensate for some wear. The COP ignitions have plenty of power to create a spark when the plugs are even .070" or more.
Last fall I changed the plugs in an 03' 4.0l and replaced the original plugs with over 60k miles on them, the gap may have opened up .010" and those were single platinums.
Different sets of single platinums are what is from the factory. The plugs on one side have platinum plated electrodes, while the other side has platinum plated tips. The single platinums you buy in stores are all the same. also Ford saves money by buying hundreds of thousands of single platinum plugs. After retail markups, those savings to the consumer would be almost nothing.
I have never had the alleged problems with double platinums that you describe. The extra stress from excessive gaps is what takes down coil packs.
As mentioned and backed up by several people on this forum with experience, use the Autolite Double Platinums, that way you don't have to figure out or worry which plugs go on which side.
BTW, the double platinums in my Dad's Dakota lasted over 200,000 miles. The single platinums that were used as replacements lasted 15,000 miles. Both plug sets were Autolite.
Also, if you would read the original post, you would see that the owners manual calls for a double platinum plug. For the record, Autolite is a Ford company, and manufactures the Motorcraft brand plugs.