s = 'heelohellohellohellohellodsdgfsh'
#字符串拼接in或者not in
new_s=''
for item in s:
    if item not in new_s:
        new_s+=item
print(new_s)


#索引+noy in
new_s2=''
for i in range(len(s)):
    if s[i] not in new_s2:
        new_s2+=s[i]
print(new_s2)


#通过集合去重
new_s3=set(s)
lst=list(new_s3)
lst.sort(key=s.index)
print(''.join(lst))

（仅供参考）