Say that you have some-addon and other-addon, and both have a foo service, and both export app/services/foo.js.
What you would observe is that other-addon would win over some-addon providing the foo service in your consuming app, because modules are resolved alphabetically.
To get around this, you can essentially proxy the services in your app folder, like so:
// app/services/foo-a.js
export { default } from 'some-addon/services/foo';
// app/services/foo-b.js
export { default } from 'other-addon/services/foo';
Then consume in your app with the new name:
// app/component/foo-bar.js
...
fooA: Ember.inject.service('foo-a'),
fooB: Ember.inject.service('foo-b')