router reuse error

[keyiis]

"koa": "^2.7.0"
"koa-router": "^7.4.0",
"typescript": "^3.5.2",
"@types/koa": "^2.0.48",
"@types/koa-router": "^7.0.40",
I define nest router,like:

c_router.post('/test',async ctx => {
ctx.body = 'success';
});
d_router.post('/test',async ctx => {
ctx.body = 'success';
});

// c_router was used twice
a_router.use('/c',c_router.routes());
b_router.use('/c',c_router.routes());

router.use('/a',a_router.routes());
router.use('/b',b_router.routes());

when post to http://xxx/a/c/test,get Not Found,if i modify b_router.use('/c',c_router.routes()); to b_router.use('/c',d_router.routes()); ,all work fine.so route(c_router) can not reuse?