swarming: log warning insated of exception for duplicated request_uuid
swarming: log warning insated of exception for duplicated request_uuid

There is a case where requests are retried with different task
attribute (e.g. timestamp in some properties crbug.com/1052144#c19).
crbug.com/1052144#c23

Bug: 1052144
Change-Id: I71ca5d54d8339d0384b9b1939fc554d41d82c0ec
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2093454
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py