F = open ('24-s1.txt')
c = cc = 0
for line in F:
    if 'YZ' in line and 'YZ' in line.replace('YZ', '', 1): c += 1
    if line.count('YZ') > 1: cc += 1
print (c, cc)

  
>>> def f(text, sub):
...     pos = text.find(sub)
...     if pos >= 0:
...         return text.find(sub, pos + len(sub)) >= 0
...     else:
...         return False
... 
>>> f('a', 'a')
False
>>> f('aa', 'a')
True
>>> f('abcdefghiabcdef', 'abc')
True
>>> f('abcdefghiabcdef', 'def')
True
>>> f('abcdefghiabcdef', 'ghi')
False
>>> 
>>> def g():
...     lines = [
...         'abcdefghiabcdefghi',
...         'abcdefghidefghi',
...         'defghiabcdefghi',
...         'abcabc'
...     ]
...     counted = 0
...     for i in lines:
...         counted += f(i, 'abc')
...     print(counted)
... 
>>> g()
2
>>>