If you install ember-cli-fastboot 4.1.5, you will always get two different versions of fastboot in your dependency graph:

ember-cli-fastboot 4.1.5
├── fastboot 4.1.5
└─┬ fastboot-express-middleware 4.1.2
  └── fastboot 4.1.2

This is because workspace:* dependencies become exact version matches when published.

It would probably be better to use workspace:^.