UDP sendto() sends duplicate packets

[AndyPapageorgiou]

BPO	37542
Nosy	@agronholm, @websurfer5

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-07-10.10:51:03.608>
labels = ['3.7', 'type-bug', 'library']
title = 'UDP sendto() sends duplicate packets'
updated_at = <Date 2019-12-27.22:17:13.739>
user = 'https://bugs.python.org/AndyPapageorgiou'

bugs.python.org fields:

activity = <Date 2019-12-27.22:17:13.739>
actor = 'alex.gronholm'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-07-10.10:51:03.608>
creator = 'Andy Papageorgiou'
dependencies = []
files = []
hgrepos = []
issue_num = 37542
keywords = []
message_count = 2.0
messages = ['347614', '358919']
nosy_count = 3.0
nosy_names = ['alex.gronholm', 'Jeffrey.Kintscher', 'Andy Papageorgiou']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37542'
versions = ['Python 3.7']

[AndyPapageorgiou]

Wireshark reports two identical back to back packets sent for each sendto(), timing between packets is between 2 and 10us.

Note this is on a point to point ethernet (just 1 cable, no switches, routers or anything else in between) to an embedded platform (Zynq ARM) from an intel i7 Window 10 laptop from 2018. The python code is running on the laptop.

python code:
with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as s:
s.bind(('', 5704))
while(more_to_send == TRUE)
<code to populate bytes_to_send>
s.sendto(bytes_to_send, (HOST, UDP_PORT))
data = s.recv(1024)

Wireshark log.
43204 80.146000 192.168.1.20 192.168.1.10 UDP 566 5704 → 5604 Len=524 (payload)
43205 80.146003 192.168.1.20 192.168.1.10 UDP 566 5704 → 5604 Len=524 (duplicate at payload time + 3us)
43206 80.149112 192.168.1.10 192.168.1.20 UDP 48 5604 → 5704 Len=6 (ack)
43207 80.149306 192.168.1.20 192.168.1.10 UDP 566 5704 → 5604 Len=524 (payload)
43208 80.149311 192.168.1.20 192.168.1.10 UDP 566 5704 → 5604 Len=524 (duplicate at payload time +5us)

I am suspicious this is an artefact of the point to point link.

[agronholm]

Can you reproduce this on localhost, or over Ethernet while only listening on that specific interface? If not, then likely this is just a Wireshark artifact.

Failing that, you should construct a script that allows others to try to reproduce the effect.