Don't use variable whose names differ only by case.

Change the collation on the question_attempt_step_data table specifically
ALTER TABLE mdl_question_attempt_step_data CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin;
(change mdl_ to the right prefix for your DB. This will not cause any unwanted side-effects.

You need to use Mysql database with case-insensitive collation (for example utf8_general_ci )

1. MASTER: Run unittests from lib/dml/tests/dml_test.php, the test test_sql_equal() should pass (no matter there is an unrelated failure with test_unique_index_collation_trouble()).
2. ALL VERSIONS: Create a course, open question bank
3. Add calculated question as in example https://docs.moodle.org/31/en/Calculated_question_type
4. Add another calculated question but use the variables that differ only in the case, for example

   {x} and {X}

5. Preview question, edit, duplicate
6. Create quiz with this question and attempt this quiz
7. Make sure there are no errors anywhere

On MSSQL (Master only)

1. Run unittests from lib/dml/tests/dml_test.php
2. Repeat with some CI or AI collation, again test test_sql_equal() should pass

On all databases:

1. Verify after install AND upgrade that there isn't any unique index on question_attempt_step_data with the columns 'attemptstepid' AND 'name' ('attemptstepid' non-unique index alone is ok).