In C I used to make it so my standard per-file and per lib code could be cut and pasted to other files/libs without modification. (E.g. every file had a mLocal variable that was file-visibility symbols, every module had a #module define for logging, there was always a mLocal.stats member, etc. ) I think some of this duplicate vs. abstract depends on your languages expressiveness - Rust or Lisp with good compile type power make it possible to squeeze out a lot of duplication that in less expressive languages are just idioms - here’s the five lines to make a syscall, or here’s the skeleton of parsing a portable network buffer into a native object.

Having a lot of if/else in your code is definitely a cost. My weakness isn’t so much the libraries and APIs, but the actual binary - once I have a service that does A very well, and I run into needing A’ I mostly just add in a config line “op_mode = A|A’” and have the else/if chains in the server driving code. Moreso for CLIs that I use myself than production services, but I have added tunables for consistency and replication to datastores to allow new use cases and expand my footprint in the data center.