It is so easy, for claiming refund now in this situation you have to file your return once again. Since you have mentioned in your question that you have filled your return and in a tax paid column you have just mentioned only one tax payment. But in reality you should have filled the both of challan's payment.
Don't Worry about it, You can easily get your refund. The following Procedure you will have to follow:
1. First go to website of Income tax department and download ITR-1 in excel format.
2. After that open the same, and filled your information about income. (In type of Return, Choose Revised Return)
3. and in last when you will fill the information about your Self Assessment Tax, Mention your both of challan details. i.e. challan number, BSR code, date of payment and amount of tax.
4. Then make a XML file by clicking generate XML button and save it in your desktop.
5. Once again login with your id and password.
6. Select Return Filling section or Upload Return.
7. After that file your return and got acknowledgement of the same.
8. You can verify it by your aadhar OTP.
9. You can sent it CPC Bangalore after signed the acknowledgement.
10. You can submit the same with your digital signature.
After a few days, your refund will be credited in your bank account.