I gave it a shot but array formulas don't work well with concatenate, so it's a mess.
<b>Excel 2003</b><table cellpadding="2.5px" rules="all" style=";background-color: #FFFFFF;border: 1px solid;border-collapse: collapse; border-color: #A6AAB6"><colgroup><col width="25px" style="background-color: #E0E0F0" /><col /><col /></colgroup><thead><tr style=" background-color: #E0E0F0;text-align: center;color: #161120"><th></th><th>A</th><th>B</th></tr></thead><tbody><tr ><td style="color: #161120;text-align: center;">1</td><td style=";">Golf</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">2</td><td style=";">Echo</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">3</td><td style=";">Golf</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">4</td><td style=";">Bravo</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">5</td><td style=";">Echo</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">6</td><td style=";">Delta</td><td style=";">Delta,Delta,Delta</td></tr><tr ><td style="color: #161120;text-align: center;">7</td><td style=";">Delta</td><td style=";">Delta,Delta</td></tr><tr ><td style="color: #161120;text-align: center;">8</td><td style=";">Delta</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">9</td><td style=";">Charlie</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">10</td><td style=";">Golf</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">11</td><td style=";">Alpha</td><td style=";">Alpha,Alpha,Alpha</td></tr><tr ><td style="color: #161120;text-align: center;">12</td><td style=";">Alpha</td><td style=";">Alpha,Alpha</td></tr><tr ><td style="color: #161120;text-align: center;">13</td><td style=";">Alpha</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">14</td><td style=";">Charlie</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">15</td><td style=";">Foxtrot</td><td style=";">Foxtrot,Foxtrot</td></tr><tr ><td style="color: #161120;text-align: center;">16</td><td style=";">Foxtrot</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">17</td><td style=";">Delta</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">18</td><td style=";">Alpha</td><td style=";">-</td></tr><tr ><td style="color: #161120;text-align: center;">19</td><td style=";">Golf</td><td style=";">Golf,Golf</td></tr><tr ><td style="color: #161120;text-align: center;">20</td><td style=";">Golf</td><td style=";">-</td></tr></tbody></table><p style="width:3.6em;font-weight:bold;margin:0;padding:0.2em 0.6em 0.2em 0.5em;border: 1px solid #A6AAB6;border-top:none;text-align: center;background-color: #E0E0F0;color: #161120">Sheet1</p><br /><br /><table width="85%" cellpadding="2.5px" rules="all" style=";border: 2px solid black;border-collapse:collapse;padding: 0.4em;background-color: #FFFFFF" ><tr><td style="padding:6px" ><b>Worksheet Formulas</b><table cellpadding="2.5px" width="100%" rules="all" style="border: 1px solid;text-align:center;background-color: #FFFFFF;border-collapse: collapse; border-color: #A6AAB6"><thead><tr style=" background-color: #E0E0F0;color: #161120"><th width="10px">Cell</th><th style="text-align:left;padding-left:5px;">Formula</th></tr></thead><tbody><tr><th width="10px" style=" background-color: #E0E0F0;color: #161120">B1</th><td style="text-align:left">=IF(<font color="Blue">A2=A1,A1&","&A2,"-"</font>)&IF(<font color="Blue">AND(<font color="Red">A2=A1,A3=A2</font>),","&A3,""</font>)&IF(<font color="Blue">AND(<font color="Red">A2=A1,A3=A2,A4=A1</font>),","&A4,""</font>)&IF(<font color="Blue">AND(<font color="Red">A2=A1,A3=A2,A4=A3,A5=A4</font>),","&A5,""</font>)</td></tr></tbody></table></td></tr></table><br />
