diff --git a/backend/src/services/contract.service.ts b/backend/src/services/contract.service.ts
index 7e3ee8a..d14102f 100644
--- a/backend/src/services/contract.service.ts
+++ b/backend/src/services/contract.service.ts
@@ -653,6 +653,15 @@ export async function batchRenew(orgId: string, userId: string, contractIds: str
     const newEndDate = new Date(newStartDate)
     newEndDate.setFullYear(newEndDate.getFullYear() + years)
 
+    // 将旧合同 endDate 截止到续签开始日前一天，避免重叠
+    const oldEndDate = new Date(newStartDate)
+    oldEndDate.setDate(oldEndDate.getDate() - 1)
+
+    await prisma.laborContract.update({
+      where: { id: contract.id },
+      data: { endDate: oldEndDate },
+    })
+
     await prisma.laborContract.create({
       data: {
         orgId,
@@ -682,6 +691,19 @@ export async function addContract(orgId: string, userId: string, data: any) {
     throw { code: 'NOT_FOUND', message: '员工不存在' }
   }
 
+  // 检查是否存在日期完全相同的合同，防止重复提交
+  const duplicate = await prisma.laborContract.findFirst({
+    where: {
+      employeeId: data.employeeId,
+      orgId,
+      startDate: new Date(data.startDate),
+      ...(data.endDate ? { endDate: new Date(data.endDate) } : { endDate: null }),
+    },
+  })
+  if (duplicate) {
+    throw { code: 'DUPLICATE', message: '该员工已存在相同日期的合同，请勿重复添加' }
+  }
+
   const contractMonths = data.endDate
     ? Math.ceil(daysBetween(new Date(data.endDate), new Date(data.startDate)) / 30.44)
     : data.contractYears * 12