mixin.bugfix.buffer_builder_leak and https://github.com/FxMorin/MemoryLeakFix/blob/1.19/src/main/java/ca/fxco/memoryleakfix/mixin/customPayloadLeak/MinecraftClient_freeBufferMixin.java
mixin.perf.remove_biome_temperature_cache and https://github.com/FxMorin/MemoryLeakFix/blob/1.19/src/main/java/ca/fxco/memoryleakfix/mixin/biomeTemperatureLeak/Biome_threadLocalMixin.java
mixin.perf.compact_bit_storage and https://github.com/FxMorin/MemoryLeakFix/blob/1.19/src/main/java/ca/fxco/memoryleakfix/mixin/spongePoweredLeak/Main_clientLoadedMixin.java

Not sure if there's more. https://github.com/fxmorin/MemoryLeakFix/wiki/Current-Fixes#both
Is it really necessary to use memory leak fix mod if one already installed modernfix?

The only patch that these mods have in common is remove_biome_temperature_cache, and it is generally harmless to have multiple versions of that patch applied. The other two patches are not the same.

As for the other fixes, it depends on whether your mods are affected by these memory leaks. Personally, I have never needed that mod, but your mileage may vary.