Added a section on the same expression being used multiple times in the same f-string.

pep-0498.txt

@@ -640,10 +640,45 @@ having 2 expressions::
 
   f'{x:.{width}}'
 
-Expressions used multiple times
--------------------------------
+The same expression used multiple times
+---------------------------------------
 
-xxx
+Every expression in braces in an f-string is evaluated exactly
+once. If the same expression is used more than once in the same
+f-string, it will be evaluated multiple times. However, it's undefined
+which result will show up in the resulting string value. For purposes
+of this section, two expressions are the same if they have the exact
+same literal text defining them. For example, '{i}' and '{i}' are the
+same expression, but '{i}' and '{ i}' are not, due to the extra space
+in the second expression.
+
+For example, given::
+
+  >>> def fn(lst):
+  ...    lst[0] += 1
+  ...    return lst[0]
+  ...
+  >>> lst=[0]
+  >>> f'{fn(lst)} {fn(lst)}'
+  '1 2'
+
+The resulting f-string might have the value '1 2', '2 2', '1 1', or
+even '2 1'.
+
+However::
+
+  >>> lst=[0]
+  >>> f'{fn(lst)} { fn(lst)}'
+  '1 2'
+
+This f-string will always have the value '1 2'. This is due to the two
+expressions not being the same: the space in the second example makes
+the two expressions distinct.
+
+This restriction is in place in order to allow for a possible future
+extension allowing translated strings, wherein the expression
+substitutions would be identified by their text values as they show up
+between the braces.
 
 References
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