Fix issue-stopper when adding a duplicate in a WS

http://sentry.bikalabs.com/bikalims/bikalims/group/619/
1399286388.830.202492715254 http://demo.bikalabs.com/worksheets/WS14-028/listing_string_entry
Traceback (innermost last):
  Module ZPublisher.Publish, line 138, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 48, in call_object
  Module bika.lims.browser.calcs, line 358, in __call__
  Module bika.lims.browser.calcs, line 317, in calculate
  Module bika.lims.browser.duplicateanalysis, line 43, in __call__
TypeError: float() argument must be a string or a number