I came across a duplicate GIDNumber value in AD Groups. This is a hidden attribute, so can't be viewed/edited using the AD Users and Groups MMC.
Querying AD
This example shows the main attributes, but not the one we are after
PS C:\Users\administratoor> Get-ADGroup -Identity "Sales Maximus Team"
DistinguishedName : CN=Sales Maximus Team,OU=Sales,OU=Castle Hill,OU=Distribution,OU=Groups,OU=APAC,DC=sydney,DC=flowerpots,DC=com
GroupCategory : Security
GroupScope : Universal
Name : Sales Maximus Team
ObjectClass : group
ObjectGUID : aabbcc55-1122-3bba-aadf-a663eeec2341
SamAccountName : Sales Maximus Team
SID : S-1-5-21-1256578901-1546006358-1122334201-11042
This example now includes the GIDNumber
PS C:\Users\administratoor> Get-ADGroup -Identity "Sales Maximus Team" -properties GIDNumber
DistinguishedName : CN=Sales Maximus Team,OU=Sales,OU=Castle Hill,OU=Distribution,OU=Groups,OU=APAC,DC=sydney,DC=flowerpots,DC=com
GIDNumber : 11224
GroupCategory : Security
GroupScope : Universal
Name : Sales Maximus Team
ObjectClass : group
ObjectGUID : aabbcc55-1122-3bba-aadf-a663eeec2341
SamAccountName : Sales Maximus Team
SID : S-1-5-21-1256578901-1546006358-1122334201-11042
Search for that GID Number in AD
I can query AD Groups for all matching records.
PS C:\Users\administratoor> Get-ADGroup -filter 'GIDNumber -eq 11224' -properties GIDNumber
DistinguishedName : CN=Finance AR,OU=Groups,OU=Disabled,OU=APAC,DC=sydney,DC=flowerpots,DC=com
GIDNumber : 11224
GroupCategory : Security
GroupScope : Universal
Name : Finance AR
ObjectClass : group
ObjectGUID : aabbcc55-1122-3bba-aadf-a663eeec2342
SamAccountName : Finance AR
SID : S-1-5-21-1256578901-1546006358-1122334201-11043
DistinguishedName : CN=Sales Maximus Team,OU=Sales,OU=Castle Hill,OU=Distribution,OU=Groups,OU=APAC,DC=sydney,DC=flowerpots,DC=com
GIDNumber : 11224
GroupCategory : Security
GroupScope : Universal
Name : Sales Maximus Team
ObjectClass : group
ObjectGUID : aabbcc55-1122-3bba-aadf-a663eeec2341
SamAccountName : Sales Maximus Team
SID : S-1-5-21-1256578901-1546006358-1122334201-11042
Look for a new GID (one which isn't used)
There are ways of looping etc, I've just used an example to find an unused one manually.
PS C:\Users\administratoor> Get-ADGroup -filter 'GIDNumber -eq 11225'
Modify the existing group's GIDNumber
PS C:\Users\administratoor> Set-ADGroup -identity "Sales Maximus Team" -Replace @{"GIDNumber" = "11225"}
Confirm the change was successful
PS C:\Users\administratoor> Get-ADGroup -filter 'GIDNumber -eq 11225'
DistinguishedName : CN=Sales Maximus Team,OU=Sales,OU=Castle Hill,OU=Distribution,OU=Groups,OU=APAC,DC=sydney,DC=flowerpots,DC=com
GroupCategory : Security
GroupScope : Universal
Name : Sales Maximus Team
ObjectClass : group
ObjectGUID : aabbcc55-1122-3bba-aadf-a663eeec2341
SamAccountName : Sales Maximus Team
SID : S-1-5-21-1256578901-1546006358-1122334201-11042
PS C:\Users\administratoor> Get-ADGroup -Identity "Sales Maximus Team" -properties GIDNumber
DistinguishedName : CN=Sales Maximus Team,OU=Sales,OU=Castle Hill,OU=Distribution,OU=Groups,OU=APAC,DC=sydney,DC=flowerpots,DC=com
GIDNumber : 11225
GroupCategory : Security
GroupScope : Universal
Name : Sales Maximus Team
ObjectClass : group
ObjectGUID : aabbcc55-1122-3bba-aadf-a663eeec2341
SamAccountName : Sales Maximus Team
SID : S-1-5-21-1256578901-1546006358-1122334201-11042