Pick source's default distance rate by index when cloning for duplication

The server-side DUPLICATE_POLICY assigns the API-known customUnitRateID
to the source's default rate (lowest index, matching getDefaultMileageRate).
Previously the optimistic clone picked the first enabled rate by Object.values
iteration order, which can differ from index order when rates are reordered
or returned in non-index order. The mismatch caused offline-created distance
expenses to silently change rate (and tax) once the server response merged
back into Onyx.

Sort by index ascending, pick the first enabled, and bind the API-known
customUnitRateID to that rate so optimistic and server data align on the
same key.

Author: wildan-m

src/libs/PolicyUtils.ts

@@ -309,17 +309,19 @@ function hasEligibleActiveAdminFromWorkspaces(policies: OnyxCollection<Policy> |
 
 function cloneCustomUnitWithNewIDs(unit: CustomUnit, newCustomUnitID: string, newDefaultRateID?: string): CustomUnit {
     if (newDefaultRateID) {
-        // For distance units: use the provided rate ID for the default rate,
-        // matching the customUnitRateID sent to the DUPLICATE_POLICY API
-        const defaultRate = Object.values(unit.rates).find((rate) => rate.enabled) ?? Object.values(unit.rates).at(0);
+        // The server-side DUPLICATE_POLICY assigns newDefaultRateID to the source's default rate
+        // (lowest index, matching getDefaultMileageRate). Mirror that here so the optimistic data
+        // doesn't get overwritten when the server response merges. Other source rates get fresh
+        // server IDs, so we drop them from the optimistic state to avoid stale duplicates.
+        const sortedRates = Object.values(unit.rates).sort((a, b) => (a.index ?? Number.MAX_SAFE_INTEGER) - (b.index ?? Number.MAX_SAFE_INTEGER));
+        const defaultRate = sortedRates.find((rate) => rate.enabled !== false) ?? sortedRates.at(0);
         return {
             ...unit,
             customUnitID: newCustomUnitID,
             rates: defaultRate ? {[newDefaultRateID]: {...defaultRate, customUnitRateID: newDefaultRateID}} : {},
         };
     }
 
-    // For other units (per diem): only update the customUnitID
     return {
         ...unit,
         customUnitID: newCustomUnitID,

tests/unit/PolicyUtilsTest.ts

@@ -354,6 +354,53 @@ describe('PolicyUtils', () => {
                 getCustomUnitsForDuplication(policyWithoutCustomUnits, true, true, {distanceCustomUnitID: otherUnit.customUnitID, perDiemCustomUnitID: perDiemUnit.customUnitID}),
             ).toBeUndefined();
         });
+
+        it('clones the source default rate (lowest enabled index) under the API-known customUnitRateID', () => {
+            const distanceUnitWithMultipleRates = {
+                customUnitID: 'srcDist',
+                name: CONST.CUSTOM_UNITS.NAME_DISTANCE,
+                enabled: true,
+                attributes: {unit: CONST.CUSTOM_UNITS.DISTANCE_UNIT_MILES, taxEnabled: true},
+                rates: {
+                    rateB: {customUnitRateID: 'rateB', name: 'New Rate 1', rate: 100, currency: 'USD', enabled: true, index: 1, attributes: {taxRateExternalID: 'tax_other'}},
+                    rateA: {customUnitRateID: 'rateA', name: 'Default Rate', rate: 70, currency: 'USD', enabled: true, index: 0, attributes: {taxRateExternalID: 'tax_default'}},
+                },
+            };
+            const policyWithMultipleRates: Policy = {
+                ...createRandomPolicy(0),
+                customUnits: {[distanceUnitWithMultipleRates.customUnitID]: distanceUnitWithMultipleRates},
+            };
+            const result = getCustomUnitsForDuplication(policyWithMultipleRates, true, false, {distanceCustomUnitID: 'newDist', perDiemCustomUnitID: 'newPerDiem', customUnitRateID: 'newRate'});
+            expect(result).toEqual({
+                newDist: {
+                    ...distanceUnitWithMultipleRates,
+                    customUnitID: 'newDist',
+                    rates: {
+                        newRate: {customUnitRateID: 'newRate', name: 'Default Rate', rate: 70, currency: 'USD', enabled: true, index: 0, attributes: {taxRateExternalID: 'tax_default'}},
+                    },
+                },
+            });
+        });
+
+        it('falls back to the first rate when no enabled rate exists', () => {
+            const distanceUnitAllDisabled = {
+                customUnitID: 'srcDist',
+                name: CONST.CUSTOM_UNITS.NAME_DISTANCE,
+                enabled: true,
+                attributes: {unit: CONST.CUSTOM_UNITS.DISTANCE_UNIT_MILES},
+                rates: {
+                    rateA: {customUnitRateID: 'rateA', name: 'Disabled', rate: 50, currency: 'USD', enabled: false, index: 0},
+                },
+            };
+            const policyAllDisabled: Policy = {
+                ...createRandomPolicy(0),
+                customUnits: {[distanceUnitAllDisabled.customUnitID]: distanceUnitAllDisabled},
+            };
+            const result = getCustomUnitsForDuplication(policyAllDisabled, true, false, {distanceCustomUnitID: 'newDist', perDiemCustomUnitID: 'newPerDiem', customUnitRateID: 'newRate'});
+            expect(result?.newDist.rates).toEqual({
+                newRate: {customUnitRateID: 'newRate', name: 'Disabled', rate: 50, currency: 'USD', enabled: false, index: 0},
+            });
+        });
     });
     describe('getRateDisplayValue', () => {
         it('should return an empty string for NaN', () => {