I used to collect and have mutiples but since I've stated using the only keep what I use approach. That stated I have 2 dr.s dd delays on my board and I can see myself adding a third. If you know what you like, and you know it would be beneficial to have more than one in different orders its not silly at all to have mutiples.
If you are a working musician, yes it's very reasonable. WHen you play a lot of shows, when your pedals leave your bedroom more than once a year, yes. In a nutshell **** happens and if you use say a Tim that is no longer made, etc or a pedal on a waiting list you might want to have a backup or at least something similar. It's practical lots of touring musicians have multiple guitars, amps, pedals that are exactly the same. It's not a new concept.
I have three boards. A gigging board, which is at rehearsal, and is far too big and heavy, to transport back home, every week.
Then there's the home board, which isnt a replica, but a close aproximation of the gigging board, build out of surplus pedals. This is for home practice and recording.
The third board is a jam board, with just three pedals, which is something I just can grab and go to a friends house to jam. Again this is made out of surplus pedals, which isnt as refined as the big board, but still fun.
You do what you like. There are no rules. Most of my pedals are cheaper than my wifes shoes.
Jon Herington (Steely Dan) talks about using multiple RC Boosters on his pedal board. I like the idea of using one to compensate for EQ changes between guitars/pickups. I have a '53 relic tele and the bridge pickup is considerably brighter and hotter than the other two pickups.
On the subject of being unreasonable, I bought two RC boosters this week.
I tend to gravitate toward basic designs and have 'similar' backups of my bread & butter items. I have a few modulation peds, a wah, and a volume, but most of my stuff is either gain/dirt or delay oriented. I tend to have exact duplicates of utility items: I have three Boss NS-2s, two Boss TU-2s and one TU-3, and four Snark clip-on tuners. I do have a 2003 Timmy and a 2013 Timmy, and I guess I have three E-bows stored in different gig bags/boxes.
I've always had two tube screamers. On my big layout, I have a Distortion+, a Barber GC, and a TS10. There is a TS808 in front with high output and very low gain that I use to boost the lead with any of the following pedals.