fin = 'a.txt'
fout = 'b.txt'
with open(fin) as f:
ss = f.reads().strip().split('\n')
s2 = [ss[0]]
for s in ss[1:]:
if s != s2[-1]:
s2.append(s)
with open(fout, 'w') as f:
f.write('\n'.join(s2))
or
fin = 'a.txt'
fout = 'b.txt'
with open(fin) as f:
ss = f.reads().strip().split('\n')
s2 = [ss[i] for i in range(len(ss)) if i==0 or ss[i]!=ss[i-1]]
with open(fout, 'w') as f:
f.write('\n'.join(s2))