Company is pushing me to take job saying they won't even hold the offer for me less than a week after getting the offer

I got a job offer from my current company that I've been working temporarily for. Of course over the phone I didn't accept the job right away and asked time to consider the offer, my prospective manager said "sure" and was pretty sure I had a standard week. I emailed HR to let them know that there was one other company that I'd been interviewing for and would try and speed up the process as to respect their deadline. I asked HR if I could let them know this Tuesday, so technically 2 extra days. She said it was fine but they are continuing to pursue other candidates and there is no guarantee I would even still have the position when I signed. I'm confused, hurt and pretty mad about this. The HR person won't even guarantee me the standard one week decision time and is making it sound like I never even had the job even though I got the offer. Isn't it standard to offer the job, give the person a week and then start asking other candidates?

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