Sum for distinct employee using xslt

I have a xml as below:

<root>
   <row>
      <Leave_days>6</Leave_days>
      <Maximum>10</Maximum>
      <Employee>John</Employee>
   </row>
   <row>
      <Leave_days>4</Leave_days>
      <Maximum>15</Maximum>
      <Employee>Albert</Employee>
   </row>
   <row>
      <Leave_days>2</Leave_days>
      <Maximum>10</Maximum>
      <Employee>John</Employee>
   </row>
</root>

I need to sum the 'Maximum' but not consider the repeating employee. So for the example above, the output should be 25 (10 for John and 15 for Albert - should ignore the 2nd row for John as it is repeating)

<Root>
   <row>25</row>
</Root>

In the below xslt, I'm not sure how to add a condition to eliminate the repeating employee row:

<xsl:template match="root">
    <Root>            
        <row>
            <xsl:value-of select="sum(row/Maximum)"/>
         </row>          
    </Root>
</xsl:template>

Also tried with group-by but not able to achieve the result. Please help.

I believe the simplest solution - even in XSLT 2.0 - would be to use the Muenchian grouping expression to select the distinct employee rows:

<xsl:stylesheet version="1.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>

<xsl:key name="row-by-emp" match="row" use="Employee" />

<xsl:template match="/root">
    <Root>            
        <row>
            <xsl:value-of select="sum(row[count(. | key('row-by-emp', Employee)[1]) = 1]/Maximum)"/>
         </row>          
    </Root>
</xsl:template>

</xsl:stylesheet>

To do this with XSLT 2.0 group-by you'd need something like:

<xsl:stylesheet version="2.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>

<xsl:template match="/root">
    <xsl:variable name="maxima">
        <xsl:for-each-group select="row" group-by="Employee">
            <xsl:copy-of select="Maximum"/>
        </xsl:for-each-group>
    </xsl:variable>
    <Root>            
        <row>
            <xsl:value-of select="sum($maxima/Maximum)"/>
         </row>          
    </Root>
</xsl:template>

</xsl:stylesheet>