What does 'shadows' mean?

[Graham Samuel]

Working on a script I've been using for some time, I just got a compilation error for the local definition of a variable called "t1". The error was:

local: name shadows another variable or constant

The only meaning I can put to this is that I had already defined this variable either as a local, global or constant in the current script - but searching the script shows that I haven't. Also since I'm using explicit variables, when I delete the (only) definition of t1, I get the usual error warning me the name is undefined. So 'shadow' must mean something else.

I've tinkered around but I can't make the error go away.

Can anyone tell me what I did wrong? Doubtless it's some kind of finger trouble.

TIA

Graham