Toddler wanting to sleep in our bed -- advice please!

My daughter is 3. For the past month, she's been coming to our bed in the middle of the night to sleep. I don't know why this started -- no noticeable changes in our lives. She's had her non-crib toddler bed for almost a year. I don't know why it's become a nightly habit and I'm not sure how to break it. When we put her back, she just comes back in less than an hour. We both work and are exhausted from this. 
What should we do? Should we let her sleep with us each night when she comes? Do we continue putting her back (and losing sleep)? Is there another intervention you can think of?
Any advice and/or experiences from others would be greatly appreciated.

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