Id Software very much skirted the edge of legality by making Commander Keen outside of office hours while still employed by SoftDisk and using SoftDisk computers, and SoftDisk could have easily sued them if they wanted to. They managed to avoid that by striking a deal where the Id guys would continue to make games for SoftDisk while working on Keen and later Wolfenstein 3D.

There was a lot of code reuse between games. John Carmack is on record somewhere that the enemy navigation code from Doom and Quake still has its origins in some of the earliest 8-bit games he wrote in the 1980's.