agent: Accept duplicated ports if no other option

Commit a04fa4d4 introduced a new feature to try to have a different port for every
local candidate, even if they are on different interfaces. This breaks setups where the
application really wants a specific port and sets a range of exactly 1 port or a very small range.
In that case, if we can't find non-duplicated ports, then we just go around again and skip that check.
7 jobs for accept-duplicates-on-second-pass in 9 minutes and 11 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #3319642
build

00:00:23

passed #3319644
1809 windows docker
build msvc amd64

00:07:53

passed #3319645
1809 windows docker
build msvc x86

00:06:19

failed #3319643
1809 windows docker allowed to fail
build msys2

00:02:46

 
  Test
passed #3319648
doc-and-install

00:00:39

passed #3319646
test

00:01:05

passed #3319647
test valgrind

00:01:02

 
Name Stage Failure
failed
build msys2 Build
Git branch:   13-0-stable
GO version: go1.13.8
Built: 2020-06-01T07:45:00+0000
OS/Arch: windows/amd64
Uploading artifacts...
WARNING: build/meson-logs/: no matching files
WARNING: build/build.ninja: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 3221225477