I had faced this error recently during Dynamics AX database restoration in one of my development environment. During my restoration to over come this issue, I selected Overwrite the existing database (WITH REPLACE) in options in restoration window and restarted SQL services too but no luck.
Finally, I run below SQL query to resolve the issue.
ALTER DATABASE Your database name
SET SINGLE_USER WITH ROLLBACK IMMEDIATE;
After running above script my restoration went smoothly.
“Yours, Ravi Theja Madisetty”