Biomes are listed twice (or more times) in worldgen files

Intended. Each biome has a lot of different sets of noises where it can generate, for more variety. If this wasn't the case, the overworld biome distribution would be as boring as the nether's.

but the thing is, parameters are the same for each duplicate entry

May I ask why exactly this is intended? If the parameters are exactly the same, then the total weight of the biomes is x2, and the weight of each biome is 2x, making absolutely no change cmpared to when it is all listed only once. There's no reason to have the entries duplicated.

The parameters are not the same. Most biomes are includes twice, once with "depth": 0 and once with "depth": 1. There's also no concept of "weight" with the multi noise biome source. Adding duplicate entries changes absolutely nothing.