diff --git a/changelog/snippets/fix.6404.md b/changelog/snippets/fix.6404.md
new file mode 100644
index 0000000000..437e2ea35f
--- /dev/null
+++ b/changelog/snippets/fix.6404.md
@@ -0,0 +1 @@
+- (#6404) Fix upgrading units being duplicated when transferred after death.
diff --git a/lua/SimUtils.lua b/lua/SimUtils.lua
index c144ac2534..558206ece8 100644
--- a/lua/SimUtils.lua
+++ b/lua/SimUtils.lua
@@ -371,14 +371,14 @@ function UpgradeTransferredKennels(kennels)
 end
 
 --- Takes the units and tries to rebuild them for each army (in order).
---- The transfer procedure is fairly expensive, so it is filtered to important units (EXPs and T3 arty).
 ---@param units Unit[]
 ---@param armies Army[]
 function TransferUnfinishedUnitsAfterDeath(units, armies)
     local unbuiltUnits = {}
     local unbuiltUnitCount = 0
     for _, unit in EntityCategoryFilterDown(transferUnbuiltCategory, units) do
-        if unit:IsBeingBuilt() then
+        -- Check if a unit is an upgrade to prevent duplicating it along with `UpgradeUnits`
+        if unit:IsBeingBuilt() and not unit.IsUpgrade then
             unbuiltUnitCount = unbuiltUnitCount + 1
             unbuiltUnits[unbuiltUnitCount] = unit
         end