Can one use a summon monster spell to summon each time Bob the Celestial Monkey specifically, or must the summoned celestial monkey be a different celestial monkey each time?
The summoned creatures are always different specific creatures unless the DM is using a variant that alters the standard summon spells
The Dungeon Master's Guide on Variant: Summoning Individual Monsters says that
When a character casts a summon monster or summon nature’s ally spell, she gets a typical, random creature of the kind she chooses. As a variant in your campaign, you can rule that each spellcaster gets specific, individual creatures rather than just some random one. This variant lets players feel more ownership over the creatures that their characters summon, but it entails some special problems, so don’t allow it without considering it carefully. (37)
The text then devotes another 500 words to this variant, the reproduction of all of which probably breaks some laws. Suffice it to say, it's complicated, but it has the advantage of being officially optional if that's a meaningful bar for your group.
Summon Monster allows you to choose which kind of creature, not the specific member of that kind. Lesser Planar Ally, however, does.
Summon Monster states:
You choose which kind of creature to summon, and you can change that choice each time you cast the spell.
Lesser Planar Ally states:
If you know an individual creature’s name, you may request that individual by speaking the name during the spell (though you might get a different creature anyway).
What's the difference?
In the Spirit of Roleplaying
Would it be unreasonable for a DM to allow you to summon the same creature over and over? Probably not so unreasonable; however, that creature may get tired of being summoned by the same person over and over without being tributed in some fashion.
One thing you wouldn't want your summoned creature to think is: