Is it OK to submit the same post-doctoral fellowship application two years in a row?
I did exactly this and was successful, but I'm going to recommend against it. In my situation, I submitted essentially the same project to roughly the same range of funding bodies two years in a row. In the second year, I tweaked my proposal, I had more publications, I had a stronger CV, and I finally had my proposal accepted re-applying for a very competitive fellowship. However, I also got flat rejected from all other fellowships to which I re-applied (including those that longlisted me in the first round).

In my situation, what little feedback I received from rejections universally stressed that the project was strong but that I needed more publications to appear a suitably reliable candidate. I had only one publication for my first round of applications, but 3-4 for the second year. For the fellowship I received, the initial rejection basically said they thought the project ought to be funded but that they lacked enough funds that year and waitlisted me. I didn't get it in the end, but then when reapplying next year I was successful. As mentioned, I wasn't even longlisted for the other fellowships to which I re-applied, three of which had listed me in the first round.

It seems this will be field-dependent (I'm in the humanities) but will, as noted, depend most on the funding body. The instutition at which I finally won the fellowship was very invested in the project and wanted me to resubmit the same thing. Everywhere else was mildly interested at best.

To conclude, I'd recommend against doing this unless you have a solid reason to believe the same funding agency is likely to accept a project they rejected the first time. Having lots more publications, though helpful, is unfortunately not enough on its own.