Can I have multiple access points or repeaters with the same SSID ?

Yes, you can. There are generally three options when setting up multiple devices to be combined into a single wireless network:

1. Assign a different SSID to each access point/repeater
This can easily be setup with any WiFi hardware, the downside being that the access points form completely different networks, without auto-switching from one to the other.

2. Assign the same SSID (and login credentials), but different channels to each access point/repeater
This setup is easy to accomplish with any existing mix of WiFi hardware. Your wireless clients automatically assume that the different access points belong to the same physical network and should almost seamlessly switch between them as needed.

3. Setup a system with the same SSID, login credentials, and channel to all devices
This requires either setting up a RADIUS server for common authentication, or using some type of system from manufactures like UniFi line by Ubiquity Networks, EnGenius, or MERU Networks. This is a common setup for larger organizations with many APs.