When Sql.Param is reused, reuse the same parameter name.

src/Faithlife.Data/SqlFormatting/Sql.cs

@@ -335,7 +335,7 @@ internal override string Render(SqlContext context)
 			if (m_filter is not null)
 				filteredProperties = filteredProperties.Where(x => m_filter(x.Name));
 
-			var text = string.Join(", ", filteredProperties.Select(x => context.RenderParam(x.GetValue(m_dto))));
+			var text = string.Join(", ", filteredProperties.Select(x => context.RenderParam(key: null, value: x.GetValue(m_dto))));
 			if (text.Length == 0)
 				throw new InvalidOperationException($"The specified type has no remaining columns: {type.FullName}");
 			return text;
@@ -401,7 +401,7 @@ private sealed class JoinSql : Sql
 	private sealed class LikePrefixParamSql : Sql
 	{
 		public LikePrefixParamSql(string prefix) => m_prefix = prefix;
-		internal override string Render(SqlContext context) => context.RenderParam(context.Syntax.EscapeLikeFragment(m_prefix) + "%");
+		internal override string Render(SqlContext context) => context.RenderParam(key: this, value: context.Syntax.EscapeLikeFragment(m_prefix) + "%");
 		private readonly string m_prefix;
 	}
 
@@ -415,7 +415,7 @@ private sealed class NameSql : Sql
 	private sealed class ParamSql : Sql
 	{
 		public ParamSql(object? value) => m_value = value;
-		internal override string Render(SqlContext context) => context.RenderParam(m_value);
+		internal override string Render(SqlContext context) => context.RenderParam(key: this, value: m_value);
 		private readonly object? m_value;
 	}
 
@@ -436,7 +436,7 @@ public string Format(string? format, object? arg, IFormatProvider? formatProvide
 		{
 			if (format is not null)
 				throw new FormatException($"Format specifier '{format}' is not supported.");
-			return arg is Sql sql ? sql.Render(m_context) : m_context.RenderParam(arg);
+			return arg is Sql sql ? sql.Render(m_context) : m_context.RenderParam(key: null, value: arg);
 		}
 
 		private readonly SqlContext m_context;

src/Faithlife.Data/SqlFormatting/SqlContext.cs

@@ -13,13 +13,25 @@ public SqlContext(SqlSyntax syntax)
 
 	public DbParameters Parameters => m_parameters is null ? DbParameters.Empty : DbParameters.Create(m_parameters);
 
-	public string RenderParam(object? value)
+	public string RenderParam(object? key, object? value)
 	{
-		m_parameters ??= new List<(string Name, object? Value)>();
+		if (key is not null && m_renderedParams is not null && m_renderedParams.TryGetValue(key, out var rendered))
+			return rendered;
+
+		m_parameters ??= [];
 		var name = Invariant($"fdp{m_parameters.Count}");
 		m_parameters.Add((name, value));
-		return Syntax.ParameterPrefix + name;
+		rendered = Syntax.ParameterPrefix + name;
+
+		if (key is not null)
+		{
+			m_renderedParams ??= new();
+			m_renderedParams.Add(key, rendered);
+		}
+
+		return rendered;
 	}
 
 	private List<(string Name, object? Value)>? m_parameters;
+	private Dictionary<object, string>? m_renderedParams;
 }

tests/Faithlife.Data.Tests/SqlFormatting/SqlSyntaxTests.cs

@@ -85,6 +85,18 @@ public void FormatSql(int? id)
 		}
 	}
 
+	[Test]
+	public void SameParamTwice()
+	{
+		var id = 42;
+		var name = "xyzzy";
+		var desc = "long description";
+		var descParam = Sql.Param(desc);
+		var (text, parameters) = Render(Sql.Format($"insert into widgets (Id, Name, Desc) values ({id}, {name}, {descParam}) on duplicate key update Name = {name}, Desc = {descParam}"));
+		text.Should().Be("insert into widgets (Id, Name, Desc) values (@fdp0, @fdp1, @fdp2) on duplicate key update Name = @fdp3, Desc = @fdp2");
+		parameters.Should().Equal(("fdp0", id), ("fdp1", name), ("fdp2", desc), ("fdp3", name));
+	}
+
 	[Test]
 	public void FormatBadFormat()
 	{