assessment: verify contents of .level_group file.
assessment: verify contents of .level_group file.

Specifically: no duplicate levels; only multi and text_match levels.

rvarshney/code-dot-org,pickettd/code-dot-org,rvarshney/code-dot-org,pickettd/code-dot-org,rvarshney/code-dot-org,rvarshney/code-dot-org,pickettd/code-dot-org,pickettd/code-dot-org,pickettd/code-dot-org,pickettd/code-dot-org,pickettd/code-dot-org,rvarshney/code-dot-org,pickettd/code-dot-org,rvarshney/code-dot-org,rvarshney/code-dot-org,rvarshney/code-dot-org