I see.
For the duplicate part use conditional formatting. Select the column where the bids will be input. Go format/conditional format/highlight cell rules/duplicates. Set the cell to a contrasting colour. This will activate if a duplicate non unique entry occurs.
To keep a check on the lowest bid. Enter each bid within the same column. Click on that column and hit the sort button (Z-A).
Look here.
Or, you could use conditional formatting again. Select the column where the bids will be input. Go format/conditional format/top,bottom rules/bottom 10 items/adjust to one. Set the cell to a contrasting colour.
Sorry, it's difficult to get this over clearly. If I loose you, feel free to ask and I will knock you up a sheet.