def is_duplicate(exp):
st = Stack()
pushed = False
for c in exp:
print c
if c in close_braces and pushed is False:
st.push(c)
pushed = True
continue
if pushed is True and c not in close_braces:
st.push(c)
if pushed is True and c in close_braces:
t = st.pop()
if t in close_braces:
return True
if pushed is True and c is '(':
# flush the stack
st.flush()
pushed = False
return False
