The bug

If the objective currently in the sidebar has displayautoupdate enabled, then players with a team prefix will have their prefix displayed twice before their name in the sidebar.

How to reproduce

1. /team add team
2. /team modify team prefix "[Prefix] "
3. /team join team
4. /scoreboard objectives add score dummy
5. /scoreboard objectives modify score displayautoupdate true
6. /scoreboard objectives setdisplay sidebar score
7. /scoreboard players set @s score 1

Observed result (sidebar)

[Prefix] [Prefix] <playername>

Expected result (sidebar)

[Prefix] <playername>