```
f = ''
da = ['A', 'T', 'G', 'C', ' ']
fnn = []
print(fnn)

con = 0

x = input('Corrupted: ')
nx = list(x)

for nx in nx:
    if nx[con] in da:
        f = f + str(nx[con])
    else:
        pass

fn = f.split()

print(fn)
print(fn[0])

for i in fn:
    if fn[i] not in fnn:
        fnn = fnn.extend(fn[i])
    else:
        pass

print(fnn)

```

This script is meant to read the input then remove all letters except from `A C G T` and remove any repeating sequences that point at it, I am really struggling to get it to remove the repeating sequences, what do i need to do? what am i doing wrong? and are there any quicker ways to do this?