When we try to use PUT /milestones/{id}/backlog on https:tuleap.net (6.7.99.27) we have the following error:

{
"error": {
"code": 500,
"message": "Internal Server Error: There are duplicated ids"
},
"debug": {
"source": "MilestoneResourceValidator.class.php:179 at call stage",
"stages": {
"success": [
"get",
"route",
"negotiate",
"authenticate",
"validate"
],
"failure": [
"call",
"message"
]
}
}
}

We did not have any duplicated ids in the data sent to the server.
By the way, on the Tuleap API Explorer, the description of the message to send uses "property" instead of "id": https://tuleap.net/api/explorer/#!/milestones/updateBacklog_put