It says Duplicate Symbol because you are using twice the pointer Str1.
Note that Str1 is a pointer, not a string, and Axe is not Basic
What you can do is that:
  "STRING1"→GDB1
  "1GNIRTS"→GDB2
Then, when you need the first string, do that:
  Fill(L1,8,0)
  Copy(GDB1,L1,7)
Now, at the pointer L1, you have a null-terminated string with "STRING1"[00] in it
And to get the second string in L1, do that:
  Fill(L1,8,0)
  Copy(GDB2,L1,7)

the difference here is that, in basic, STR1 is the name of an external object that is created in ram when the program is running, and can be modified however you choose. in axe, however, STR1 is a pre-processor directive. there is no actual object named STR1 that is created, and it is not used at all by your running program. STR1 is a label used by the compiler to point to a specific spot in your compiled program itself, so you can't tell it to point to two different places at once, which is what saying "store X" -> STR1 and then "store Y" -> STR1 is doing. the first time the compiler sees that, it appends the data "store X" to your program and then labels that point as STR1 for everything else to use. the second line would, then, be trying to append data at a different spot in the program and tell the compiler that that place is also called "STR1", which is causing an issue.

now, all that being said, it's easy enough to modify data stored at an STR#, GDB#, or PIC# label later on, provided that you are compiling a program, that will run in standard RAM space, rather than an application. you have to use the label as a part of a pointer, though, and store all the number values manually. it's much easier to work with free ram areas outside of your own program, like hayleia showed above.

However, you /can/ allocate a part of the RAM as a string but that's kinda double work. (unless you have the data to be stored in the string generated by a routine)