A subpoena is a court order. If you are PROPERLY SERVED (by someone personally hand delivering you a copy of the subpoena) you are obligated to show up to court. Of course, the DA will likely represent to the Court that you HAVE been properly served, when in fact you have not, and the Judge will not only believe them, but will issue a "body attachemt" (aka an arrest warrant) for you. Only by coming to court and challenging that you were served properly can you avoid that. But then again, once you walk into court, they got you (so the challenge to your being properly served is now moot).
Can you be compelled to testify? That depends on whether or not you could incriminate yourself by what you say. If you think anything you might say from the witness stand could incriminate you, ask to speak to the Judge before being put on the stand and tell him/her your concern. If the Judge agrees, s/he might either appoint you a lawyer to reveiw your options, grant your request not to testify, or ask the DA if they intend to offer you immunity for your testimony. Either way, it sounds like you need a lawyer to represent your interests. On the other hand, if your testimony is not likely to incriminate you, but you just don't want to testify for whatever reason, yes, you can be compelled to testify. And if you once again refuse, you can be sent to jail again. Double jepordy is not an issue as each act of contempt can be punished separately.