Here is a series of steps that will fix your problem:
1. Use a better naming convention; ie, stick to camel-case. Rename your interfaces to: UserDao, UserService.
2. Even though you are autowiring UserDao twice, you still use the singleton instance created by the Spring container.
3. Remove @Qualifier annotation; it is not needed.
4. Remove line 41 from db-config.xml, since Component scanning will create this bean anyway.
5. In Web.xml remove lines 38-41. You've already created a ContextLoaderListener in lines 20-23