Master's and Bachelor's theses on the same topic?
I don't see any problem studying the same subject. You could consider it an extension of your Bachelor's thesis. As a reviewer I would welcome such a proposal, as long as the one-pager is well written and has clear statements of the problems you want to solve.
I even suggest you think about what you could have done differently in your Bachelor thesis and include it in your proposal.
Remember that you are submitting a project proposal and, during the course of your masters research, you might change your mind and decide to pursue another direction.