Error parsing .gitlab-ci.yml: Map keys must be unique "<<" is repeated

Summary

Could not parse .gitlab-ci.yml of the backend.

Steps to reproduce

Run npm i or npm run check-configuration in @openstapps/backend with @openstapps/configuration: "0.18.0" (latest).

What is the current bug behavior?

Error parsing .gitlab-ci.yml file when templates are used.

What is the expected correct behavior?

The tool should properly parse .gitlab-ci.yml file in this format.

Relevant logs and/or screenshots

Could not parse /home/jkrunic/Desktop/OpenStApps/backend/.gitlab-ci.yml because of 'Map keys must be unique; "<<" is repeated'.

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)