I haven't yet found an easy way to do this without custom code. So my only
recommendation would be to do it in code. You can loop through all the
field1-s of the repeating table in code and concatenate them.
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S.Y.M. Wong-A-Ton

Let's call the field at the bottom of the form FieldX

Rightclick on FieldX and choose Textbox properties.
In the "Default Value" box, click the Fx button, then Functions.
Choose sum.
Doubleclick on the "doubleclick here to insert field" link and find
Field1

Save and test

I assume you want to identify duplicates in field1 and create in fieldX (the
one on the bottom) a long string with all the repetitions...
On field1 in the repeating table, set a rule that has as condition the
following expression: . = ../preceding-sibling::*/my:field1  and has as
action: set a field's value: fieldX = concat(../../../my:fieldX, .)
You'll have a long string with all the repeated words;
To identify the rows that triggered the rule, add in Data Validation for
field1 the following expression: . = ../preceding-sibling::*/my:field1 (the
same that triggers the rule). The rows containing the duplicates will have a
red border.
Liviu