Python function that takes a string as parameter and returns a string with every successive repetitive character replaced

Write a Python function that takes a string as parameter and returns a string with every successive repetitive character replaced by & e.g. Parameter may become Par&met&r.

def str_encode(s):
    ans=""
    for a in s:
        if a not in ans:
            ans=ans+a
        else:
            ans=ans+'&'
    return ans

st1=input("Enter any String :")
output=str_encode(st1)
print("Output is : ",output)

Enter any String :Parameter

Output is :  Par&met&