an upcoming vs a upcoming

Some examples from the web:

• EFAS data might contribute to increased preparedness in an upcoming flood event.
• Is there an upcoming celestial event that interests you?
• What that meant was... Pete had an upcoming job and story in China which he was very, very passionate about.
• Our records indicate that you're a witness in an upcoming murder trial -
• I think my aunt has that. Mr. Nigel-Murray and I... are delivering a paper at an upcoming conference.
• Russ Brennan not only broke his parole by fleeing the region, he is a material witness in an upcoming murder trial.
• Stuart told Kenny he was doing some undercover police work for his mom and Roxanne was a witness in an upcoming murder trial.
• I've been hearing quite a bit of talk about an upcoming dance.
• For my character, that I will be essaying in an upcoming episode of TV's Santa Monica Cops, which I have agreed to do because all the finest movie stars are coming to television today.
• And stay tuned for an upcoming statement from local authorities and the FBI...
• The tarot cards tell me that you fear an upcoming event.
• Unless potatoes feature prominently in an upcoming storyline.
• Mr. Nigel-Murray and I are delivering a paper at an upcoming conference.
• She's holding a full page for you in an upcoming issue.
• As you probably know I have been cast in an upcoming feature film.
• I'm trying out a new caterer for an upcoming event.
• He wanted information about an upcoming shipment.
• And I had to get plastic surgery for an upcoming role where I play a man much more handsome than myself.
• Said there's another 20 large in it for him if he gets me to drop a tip about an upcoming gun buy.
• You're going to be deposed in an upcoming investigation, and I need our relationship to stay out of it.

Some examples from the web:

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