No import(-once) happens, if file already imported to another scope

When I use common import (without multiple), the file seems not to be imported if it was already imported into another scope.

This behavior is not obvious and produces unexpected errors. Is that a bug?

Reproduces in current version of LESS (2.7.2), and as well in upcoming 3.0.

A minimal fileset to reproduce:

variable.less

@some-variable: 'some-value';

main.less

@media all {
    @import 'variable';
}
@import 'variable';

element {
    content: @some-variable;
}

Expected CSS output:

element {
    content: 'some-value';
}

Actual output (exception):

NameError: variable @some-variable is undefined in main.less on line 7, column 14:
6 element {
7     content: @some-variable;
8 }

Start by running the minimal fileset from the issue: variable.less and main.less, using the reported LESS versions, and compare the generated CSS with the expected output. Trace how the two imports are resolved across the @media scope, then verify that the outer import makes @some-variable available without the reported exception.