Fix Issue with duplicate employee creation in addemp function Contract

[adil-techcode]

I have encountered an issue in the addemp function of the allemp contract, where duplicate employee entries are not being prevented. Upon further investigation, I found that the function currently lacks a check for existing employees. By utilizing the "require" condition, we can revert the transaction in case the employee already exists, effectively saving on gas fees. Please assign this issue to me under GSSOC'23. Thank You!