Claiming it on your return will not delay the processing and issuing of the refund. If you do not claim the $5,600 like the IRS told you to when your return is processed the IRS will deduct the $5,600 from the refund as their records show that you have received the stimulus.
My suggestion would be to report the amount as the IRS has indicated. Then get your paperwork together and contact a Taxpayer Advocate in your area. Taxpayer Advocate are there for the taxpayer and they help you resolve your issue. This link is an article about the Taxpayer Advocate Service Taxpayer Advocate.
Taxpayer Advocate Service could investigate whether or not the check was returned and not posted to your account and thus the IRS claims you received the funds.
In summary, you should claim the $5,600 to avoid delays with the processing of your return and issuance of your refund. Then contact the Taxpayer Advocate office for your area and have them assist you with getting the $5,600 check reissued.
