find.py

from collections import Counter
from collections import OrderedDict

msg = """If web server returns any error message
for any query by application server
then application server should catch and display
these error messages appropriately to the users"""

items = OrderedDict([(_, Counter(_)) for _ in msg.split()])
cleared = zip(*filter(lambda (k, v): len(v.values()) == sum(v.values()), items.iteritems()))[0]
print cleared
print cleared[0]

restult.txt

('If', 'web', 'any', 'for', 'query', 'by', 'then', 'should', 'and', 'display', 'to', 'the')
If