Include directive does not merge included variables

It doesn't look like variables defined at the YAML level are getting merged, it looks like the variable: field is just getting overwritten by the including YAML.

Behavior: .gitlab-common.yml

variables:
    SOME_COMMON_VARIABLE: test

my-job:
   script:
     - echo $SOME_COMMON_VARIABLE

.gitlab.yml

include: .gitlab-common.yml

variables:
    SOME_OTHER_VARIABLE: another-variable

$SOME_COMMON_VARIABLE is undefined in my-job

Is this behavior expected?