Why am I getting duplicate version error using db:migrate VERSION=XXX?

I'm working with an application that was upgraded from Ruby 1.8.6 to Ruby 1.8.7. I created a number of migration scripts named as 999_whatever_function.rb, ordered sequentially. The whole set from previous runs have items from 001 until 430, and I added items up to 450. When I try to use

rake db:migrate VERSION=450

It aborts with this error

C:\Ruby\projects\db_upgrade>rake db:migrate VERSION=450 --trace

** Invoke db:migrate (first_time)
** Invoke environment (first_time)
** Execute environment
** Execute db:migrate

rake aborted!

**Mu开发者_开发问答ltiple migrations have the version number 3500**

c:/Ruby/lib/ruby/gems/1.8/gems/activerecord-2.3.4/lib/active_record/migration.rb:507:in `migrations'

I have no duplicate numbering on the scripts but I'm wondering if the version 1.8.7 requires a different type of VERSION notation (YYYYMMDDTime)?

I see that you say there are no duplicate migrations, but that's what would trigger this error. Double-check the prefixes of the migrations in db/migrate. Are there two that begin with 3500?

Also, you are conflating Ruby and Rails. Ruby is at version 1.8.7, not Rails. It looks like you are running Rails 2.3.4.