Oh yes, it is normal. Surely this happens in tree cavities. Some birds excavate first, bluebirds don't usually.
I assume you usually do clean out the gourds when the martins leave? If the bluebirds are not too far along in their building, you could remove the old nest material below, leaving the new stuff so they are not discouraged.
Visit www.sialis.com for tons of info about bluebirds. And Congrats!
BlueBars

Here is a link that might be useful: http://www.sialis.org/index.html

Thanks so much BlueBars !!!....I can see the link you gave me will be very helpful.....I also thought it strange that they would be nesting at this time....maybe they had a previous nest elsewhere that was predated or destroyed in someother way ?....there is around 11 unused gourds out there with prebuilt nests for the martins.....just thought it rather odd that they would choose one that has already been used....and yeah I do clean out the nests of martins after they all leave....still have 1 nest of late arriving SY's still to fledge but I suppose I can clean the BB's gourd since its on another pole....thanks again for the info !!!

I always thought that you weren't suppose to let any other birds use a martin colony housing as that would cause the martin's to abandon the site

Here is a link that might be useful: Purple Martin website

Renee & Brenda, I found this comment (pasted below) at the Purple Martin website, but since these martins have fledged now (they only have one breeding per season, right?), so it should be OK for the bluebirds to nest there now for this year. I think this info is more for competitors in the Spring. But! The bluebirds might also return next year, so be prepared.

"Should native bird species (e.g., Tree Swallows, Eastern Bluebirds, Great Crested Flycatchers, etc.) try to take over your empty martin housing, temporarily plug all the entrance holes with door stops or paper cups, then put up appropriate, single-unit housing elsewhere on your property. Once these other birds have accepted the new housing, reopen the martin housing."

Renee & bluebars....I have been a lurker at the PMCA site for the past few years....but I thought that since the martins are basically gone from my site for the season(you're right bluebars, the martins nest only once a season) that the BB's could nest there and I will put more gourds up next year for the BB's away from the martin setups.....a flock of martins did return today for a visit and chased the BB's away from that gourd rack....i've not seen the BB's since the martins left....BB's are beautiful birds also and i would love to have them and martins in my back yard next season....i'll do whatever it takes to try and make it work......thanks for both of your helpful replies !!!!!.....This is a great forum !!!!!...Brenda

I'm new to this forum, also! ;o)
I've been trying for years to get martins & have lots of available housing up including gourds. Last year I had a pair of BB's that nested in a BB box that I placed too close to the martin setup! The male BB defended "his" territory & hunted from the martin perches...needless to say, I still don't have nesting martins...just visitors!
If I were you I'd do what Bluebars said & close up the martin gourds but I would also take down the one they are building in & put it on a shorter pole away from the martin housing. Make sure you put up predator barriers...I had netting & a squirrel baffle on my BB pole but I didn't have the netting on properly...I lost 4 of my 5 baby BB's only days from fledging to an almost 4 ft. rat snake...I was sick!!! The snake got caught after it had consumed 4 of the babies & only one female was left. I moved the house & cleaned it out after she fledged because they won't come back to the box once something like this happens. I didn't realize that I had a problem with the netting because the box had already fledged 5 chickadees a few weeks before the BB's started building their nest! I had also lost 4 mocker babies a week before the snake got the BB babies. BB's & martins are my favorite birds!!!

Best of luck to you!!!!!
They usually will nest 3-4 times in our area...I'm hoping to get them back again for their 3rd nesting!