How was "Derek Reese" alive in the future?

It is a younger pre-time-travel Derek.

A fews things about the future John Connor and Catherine Weaver travel to:

• It is a future where John Connor is not the leader of the resistance, and might not possibly even exist.
• It is the future, but before Kyle Reese travels back in time (and therefore before Derek Reese travels back)
• Derek Reese hasn't aged, he clearly isn't the same one who was shot and killed, otherwise he would be older.

"All these incidents are happening on same time line and the same earth."

That is false. In fact, we see conflicting timelines just in Season 2 with the introduction of Jesse. She has different memories of events and potentially a different Judgement Day date than Derek does. For instance, she says Derek was tortured by Charles Fischer, though Derek doesn't appear to have any memory of this.

It doesn't even need to be a younger pre-time travel Derek. At least up to this moment in the future, John Connor has been missing from history ever since the moment he departed with Weaver to travel to the future. So he would not have sent Derek back in time at all, at least not yet. So Derek is still present. A Derek died in the past, but that was a Derek that had come back from a different future as compared to the one John has created for himself by travelling forward.