Can a patent pending be published if prior art is already patented? How?

I am not sure I completely understand your question, so I will try to rephrase it so that I can answer it. It would appear that you are asking whether there are any prior art considerations or analysis that takes place at the Patent Office before a patent application is published? The answer to that question is no. US patent applications are published automatically (approximately 18 month after filing date), unless a specific "non-publication request" is filed with the original application. There is no review on the merits of whether the subject matter disclosed and claimed in any particular patent application is "patentable" before it is published. It is just published for whatever it discloses. Such a published patent application becomes prior art to any other patent applications that follow its filing date. So to answer your last question, yes, a patent application can be published even if there is a similar patent already issued. That issued patent may prevent your patent application from issuing if it is in fact relevant prior art and is considered by the examiner. But, it has no relevance on whether your pending application gets published or not.

Yes it can. Patent applications are typically published before they are examined. They are published on a schedule and so there is no comparison to the prior art during the publication process. Therefore when the patent office moves forward with publishing, the prior art is not relevant to their process.

Short answer is yes. Also, please bear in mind that it is the claims that decide what is being patented and/or whether there is an interference between two applications filed by different inventors/entities.

If a patent for a similar product has already been issued, you're likely going to have difficulty getting a patent issued for your idea. Once you submit an application of any kind to the USPTO, the information in that application is available for anyone to see on the USPTO web site.

US Patent Applications are automatically published - 18 months after the filing date - unless the applicant requests no publication - in writing at the time of filing. Thus - your application was published. The act of publication has no relationship to the examination of your application. The examiner will usually find prior art that is close to your invention. You must then amend your application claims to be both novel and non-obvious over the prior art. Once you do that - your patent can be issued. Good Luck!